First of all, let me say that I'm very sorry to hear about your misfortune. I sincerely hope that you're able to fix it at a reasonable cost.
Having said that...I feel that the comments that the seller was a "*******" is just a tad off the mark. Let's think about this. The bike is 15 going on 16 years old. The mileage was never discussed in this post, so I can't speak to that, but obviously this is not a new bike. For the seller to state "as is" seems completely normal and predictable to me. Do you seriously expect someone to guarantee a 15+ year old bike, or car for that matter, against defects or problems that aren't visible at the moment of sale? The best you can do is either arrange with the seller to have a local mechanic inspect it before you buy it, at your cost obviously, or to strike a deal with the seller that you're going to have it inspected, and the sale hinges on the result of that inspection. Beyond that, you're completely on your own.
As far as suggesting that the asking price was too high...no one forced the gentleman to purchase this bike. It's certainly the responsibility of the buyer to determine what is a fair price for the vehicle in question. Many sellers allow themselves some "haggle" room, so you almost have to try to negotiate a little, unless the original asking price is quite reasonable.
The bottom line is, when you find a vehicle you're considering purchasing...research its value, to determine what price you should expect to pay, and decide before you ever see the vehicle what price range you feel is fair. Then research known problems with that make/model and be prepared to give the vehicle a relatively in-depth inspection yourself, or at the very least take along a knowledgeable friend who can help you inspect it. Either way, you should still seriously consider having a 3rd party professional inspection done (unless the bike is virtually new,and/or in near perfect condition) to weed out any deeper issues or potential problems. The inspection shouldn't cost you any more than $50-$100 (which, as you can now see, would be money well spent), and when I purchased my '02 Triumph Sprint ST back in '05 and took it to the local Triumph dealership to have it inspected, they did it for free.
While I hope you can salvage some usefulness from this episode, just watch you don't throw away too much money on a bike that isn't worth it. If it seems too costly to fix, I'd recommend either riding it until it dies (Have you actually experienced any problems from the crack yet? Has anyone else looked at it...it isn't just an unscrupulous mechanic looking to bilk you?) or parting it out, and writing it off as a painful but valuable learning experience.
Caveat Emptor!
Good luck,
Tim