I'll add my 2 cents since I sell these systems on eBay and in my webstore and Bonanza booth.
The price paid is a factor of many variables. First & foremost, how many are available at the time of sale? When the market it flooded, the prices go down. When there is only 1 available, the price can go as high as whatever someone is willing to pay for it. That's the bottom line: an item is worth whatever someone is willing to spend for it. To me, I wouldn't give you $50.00 for the Mona Lisa, but that's just me.
When you're shopping on one of the auction sites, its all too easy to get into a bidding war so you need to be careful.
Other factors that determine the price: How clean is the system? Has it been tested? What condition is it in? Will you have to make repairs (which add to the final cost)? If it doesn't work when you get it, are you going to loose any money or can you return it for a FULL refund? What items from the original retail package are included and what is their condition?
If the system you're buying has not been cleaned thoroughly, you may be bringing rat feces & urine, dead fleshy bits of all sorts, odors, and general filth into your home. Think about all the biological agents and toxic chemicals that could be in/on your cheap system, especially if the system came from a recycler or storage facility. Even some folks' homes are disgustingly filthy.
If you purchase a cheap system and then find you have to fix it, you might wind up spending more (a lot more) for parts to get it up to snuff. I've known folks who got a dirt cheap system and, by the time they were finished buying all the replacement parts, the darn thing cost them more than they could have gotten right from the start... and saved themselves all that time to boot!
If you buy something on eBay and its not as described, you will most likely loose money, even if the seller is willing to take it back for a refund. Most sellers don't refund the cost of shipping it to you in the first place, so you will loose that money plus the cost of shipping the crap back to them. Depending on the item, it can be an expensive proposition.
There are a LOT of good buys to be had on eBay and elsewhere on line, but there are also a lot more bombs to be had. From personal experience, if you see an ad that says "AS IS", "UNTESTED", "UNKNOWN WORKING CONDITION" and the like, chances are it does NOT work. There are an awful lot of dishonest sellers who know their system does not work properly but will claim it is "untested" so that they can sell a non-working unit. If you buy anything with any of those types of disqualifiers, then assume the item is NOT working and if you get it and it is working, you're pleasantly surprised and happy with your purchase.
Finally, if you're a true collector, you're looking at the overall condition of the system AND whether or not it comes in the retail packaging (and if so, it's condition) and comes with ALL of the original retail contents, such as paperwork, cables, etc. (and their condition). Just look at the video online of the fellow from NY who recently spent $2600 for an Apple IIc in the original retail box (supposedly a NEW IN BOX system) and others who spent what may very well be considered an extravagent amount of money here in this Forum.
Bottom line is, an item is worth what YOU are willing to pay for it, and what you are willing to pay for it will depend upon which variables you consider important. If you're simply a retrocomputerist who wants to collect one of each and have them in working condition, you can get by for a lot less cash than a vintage collector, especially if you're willing to spend time thoroughly cleaning the components and making necessary repairs.