Myself, I would like a Ferrari 575M Maranello. I know they typically sell for $250k, but I only have $50k. I know about the "Super-Secret Camera Store that Sells High-Quality Gear at 80% off Every Day", so now I'm looking for the "Super-Secret Auto Dealer that Sells Italian Imports at 80% off Every Day"
I hate to say it, but lens prices are lens prices. If someone made a particular lens for 1/5 the price everyone else makes them, then that will only mean one of two things:
1. It's a crappy lens, and no one wants to buy it because it has too many optical flaws, or
2. It's a great lens, in which case it will ultimately be the only one on the market because all the others are way overpriced.
Try buying used. Maybe a local camera store has one, try
ebay, google the lens plus the phrase "For Sale", check photo auction sites, browse forums (heck, maybe someone here wants to unload theirs!), maybe rent one for a while.
Circular fisheyes are pricey simply due to their economics. Few people want one, so they have a limited sales potential. So the manufacturers will make a batch of 'em, keep them in the warehouse, and not make any more for quite a while. They don't enjoy the cost-effectiveness of popular lens with their long production runs. It costs money to retool a factory just to make a 'handful' of lenses, then sit on them hoping for a few meager sales.
BTW, the Super-Secret Camera Store that Sells High-Quality Gear at 80% off Every Day is located at the Greyhound Bus station, through the lobby, past the Men's Room, third door along. Sign on the door states, "Electrical Room". Knock three times, pause for two seconds, then cough twice. Bruno or Vinny will let you in. Tell them "Carlos the Cutter" sent you.