I paid $29 including shipping on eBay. Most for sale there are considerably higher.
Shop around for a good price.

Reviews here:
Above top secret forum  Very impressive

Factory sight
This really is a far better pistol than the 50# (some say it is really 45#) one made in China, but there have been a few shortcuts taken. The front and rear sights, the dovetail rear mount are plastic. In itself not exactly bad, but the rear sight is in my opinion crap. The slightest bump causes the rear sight to move right or left.

I removed both and replaced them with a Center Point laser sight ($29 at Walmart). My laser sight was supposed to come with self-sticking pads for the remote switch, but did not.

Assembly
Check all screws for tightness. Some were loose from the factory.
When installing the prod, I used double sided carpet tape to hold the rubber cushions in place. If you don't have the tape, use contact cement or silicone sealant. Makes assembly much easier.
The instructions recommend having a friend help when you string the prod. Even with the frame mounted in the bench vise it was not easy with two people doing it. Be careful!

There are two white lines in the prod to help get it centered in the stock, but I recommend using a rule to make sure the ends of the prod are the same distance from the frame.

The next modification was to take a file to the corners of the dovetail. They were very sharp.

The right hand side gold colored guide had a sharp edge on it. 660 grit wet or dry was used to smooth it, then 1500 grit.

Low on the priority list is the checkering on the pistol grips. A little too sharp for my taste. Some fine sandpaper will correct that. I believe the more comfortable a weapon is to hold and fire, the better you will do with it.

Shoulder stock
It is simply a piece of PVC pipe slipped over the cocking lever with a butt plate made from pine. There were several considerations before choosing the PVC pipe:
1. Drilling holes in the cocking lever would weaken it, so that was ruled out.
2. It has to be comfortable and not too much in the way when cocking the pistol.
3. A metal tube slipped over the cocking lever would gouge the plastic lever.

This is not the look I was hoping for, but it works great.
A couple of rubber bands on the PVC tube make a handy place to store spare bolts without interfering with cocking.

The tube is 1" PVC pipe, the butt is soft pine with a hardwood dowel glued in it so the screw has something to grab onto. The butt stock does not interfere with cocking, which was my greatest concern. My 87 year old father can now hold it steady. Perhaps steadier than I can. The allen wrenches for the sight and prod can be stored in the tube along with a tube of Chapstick.

For non-broadhead bolts, a couple of rubber bands were wrapped around the PVC tube to store 8 or more bolts. Simply pull them out (forward) for use. I had thought of a number of more complicated methods and may do one of those for broadhead storage.

Lubricant
The bowstring came with substantial beeswax lubricant. Cleaning that and the bolt groove with alcohol then lubricating it with Chapstick (lip balm) raised the point of impact at 15 feet by 1.5 inches. (Careful with the alcohol... it tries to remove the black paint.) Raising the point of impact tells us the velocity is greater, and penetration was a little deeper. Cocking is easier and smoother. Sliding the bolt in the groove by hand will tell you there is much less drag than there was with beeswax. Chapstick was also applied to the gold guide rails where the cocking arms slide. A tiny drop of synthetic oil was applied to the pivots on the cocking mechanism to prolong their life. UPDATE I am getting poor string life with the chapstick. While it works fine with the 50# bow, it looks like beeswax is needed for the more powerful 80# bow.

I have settled on Murray's 100% natural beeswax from Autralia as a lube. It was found in the Dollar Store in the hair-care section at a super bargain compared to commerciaL bowstring wax. It seems much thinner than beeswax I remember from beehives. But that is fine because I don't want a lot of bowstring drag. I think it is far better than Chapstick lipbalm as a lube, so I pulled the stick out of the Chapstick tube and packed it with beeswax. Much handier since I can store it in the shoulderstock tube.

Safety
The safety works in reverse of the 50# bow. I.E., pushing it forward allows it to fire. That is much handier for hunting. It operated smoothly from the factory, so no modifications were made. However, it automatically goes to the safe position every time it is cocked. I may disable that on the repeating version. Also, there is a catch for the cocking lever that has to be pressed when cocking. That would also slow down rapid fire with a repeater. I will have to think on that some more.

Bolts
The plastic bolts that are standard for the 50# bow are not recommended for the 80#. Though I can't find any such recommendation now. They work just fine and may be slightly more accurate than the ones that cam with the 80# bow. The scale tells me they are almost twice as heavy as the all aluminum ones that come with the Cobra. If you are interested in long range use these.
UPDATE I now know a good reason why not to use the cheaper all plastic bolts. The 80# bow is powerful enough to mushroom the back end of the bolt after only a few shots. That caused a few misfeeds in the new repeater when the rear of the bolts failed to drop down because the stick in the magazine follower after just 2 shots. A few bolts were destroyed in testing.
Mushrooming shown (exaggerated)

Laser sight
I used a Centerpoint Model CPO 427 sight from Walmart @ $29.  The Crosman 0420 seems similar for slightly higher ($31.90) here is one

Short Bolts

These are standard aluminum bolts shortened to 3 1/2" long. They give better accuracy, better penetration at a distance, but a little less penetration close range.

I can put quite few in my shirt pocket. The down side is they are harder to find in the field.

Note in the above photo the short bolts made a tear to the left of the hole.
This is because of skew, shown below exaggerated.

The solution is to place a strips of paper (shown in red) behind the prod to make the string at 90° to the limb. You can tell when it is correct when the bolts enter the target straight. This is important if you want maximum power and accuracy. It would be worth making a few short bolts just to make sure the string is correct.

I have worn the serving off the center of the string several times while getting the bugs and sharp corners on the pistol smoothed out. The first re-serving was 20# Dacron. Did not last very long. The second and third were of 60# braided Dacron use for fishing. So far it is great!

From the worn string serving I learned that these areas MUST be smooth.
The smoother, the better.

View looking down on the string slot.

A couple of excellent tutorials to bowstring making (and repairing the serving)
Moorabbin Archery Club
Making a Flemish twist bowstring



Ten shot 80# repeater shown with laser sight, shown without detachable shoulder stock.

More Cobra Tweaks
Shoulder Stock Quiver