"Cheap Fatsharks" are an oxymoron. LOL The cheapest new pair that you can buy, I believe, is the Dominator HDOs, at $500. You might be able to find some older versions, like the Attitude v4 or Dominator HD, but they're older models that have been discontinued and may not be able to be found easily.
People who have Fatsharks love their Fatsharks and aren't usually willing to give them up. If they are selling them, it's usually because they had the lenses exposed to sunlight and ruined, or they tried to take them apart to install something like a LaForge system and shorted something out, or something equivalent. Be cautious if you are buying a used set, know all of the features of the set you are looking to buy (for example, Teleporters don't have adjustability for eye distance; I demo'd them and got a headache; switching to a pair of goggles that offered it, the problem went away).
You can get away cheaply with other goggles, but in this case, I think you get what you pay for.