I think that it's high for a fatboy, but it all depends on the condition. I think a shaver grade fatboy can run about $20-$40 on eBay. You can post a WTB on the buy/sell/trade here on B&B and probably get a nice one for a fair price. That one appears pretty nice but it's a little pricey IMO.
You shouldn't post the thread for an ongoing auction, but I think that price is too high. Plenty come up on the BST, Ebay, or Etsy for cheaper. Many members here have sites on ebay or etsy and offer more reasonable prices. Waiting for a better price will work out best, you might even find one in the wild.
First off, make sure that what you're looking at is a genuine Fatboy. They were made from 1958~1961, date codes D~G. Many, many sellers on eBay call their razors a Fatboy, when it actually is a Slim, a Superspeed, a fat-handled Tech, or something else.
Luckily, the one you linked to is The Real Deal. The price does seem a bit high, but not by much. I paid $35 for an F-3 I bought for my brother back around 5 years ago, and prices have been steadily climbing ever since. I would say that nowadays, prices start around $50 for a user-grade Fatboy, and can go up to about $150 for one in Mint condition with case, blade and instructions.
Don't let the prices scare you. The Fatboy is a great razor, probably one of the best adjustables that Gillette ever made. But you can find the same or better performance from a lot of other razors. If you must have an adjustable, I'd recommend a Merkur Progress. For fixed head razors, your options are numerous.
Although I can't see what the pricing was at, I've seen a lot of Fatboys range from the $20 to $50 range depending on condition. +1 on dpm802's point. A lot of listings aren't actually Fatboys. My recommendation would be to put up a WTB ad in the B/S/T or go to a flea market or antique store and look around. I've seen plenty of good quality Fatboys for around $12 at flea markets.
I was able to get a Gillette superspeed today for $2.00 at an antique store in near mint condition, not the adjustable I am dying to try maybe I will stumble across one in the future.
Price is a relative thing, they can be underpriced (Mostly by neophyte who don't know their value), Overpriced by greedy seller OR Neophyte as well, over paid by bidders who don't care about the prices etc etc A 2$ SS in Minty condition is something underpriced, you got lucky that's all, like the one who purchased a NOS Toggle for 35$ or the member who got a cased Red Ring for 25$ (They were at these price because sellers were unaware of their true value). Like you maybe noticed, the DE vintage razor market is a bit on a sprint at the moment, there is new DE shavers everyday and everyone want to have their SS, Slim, FatBoy etc etc etc What decide of a price is simply Supply and Demand.
I'd say if you are buying off the net, 30ish for user grade, 40-50 for cleaner examples, more for a cased one. I'd say the earlier ones you can easily take apart command a slight premium.