Unfortunately this is fairly common. Jeanette Lee tried to buy half of herself after the fact. The bidder quiet reasonably refused. I think she has a history of that but I will stick to the one time I recall the details fairly well.
A player thinking they should get half with no risk is being ridiculous. Indeed, even ten or fifteen percent can be ridiculous depending on the real costs the bidder spent to win. Many players see this jelly as a requirement, it isn't as players find more and more when outside bidders win. The bidder might have bet tens of thousands that weekend on that calcutta, other calcuttas, horses, dogs, poker, on and on. Plus any smart gambler is keeping a running tally of where they are at for the year or longer. If they are having a great year they might be inclined to toss a little jelly but I can't picture them tossing more than a few thousand regardless of what ten or fifteen percent is.
While on the subject, unless specified in the rules for the event, the buyer isn't under obligation to sell the player 50%. Many players get out of joint about this but have no problem not buying fifty percent if they don't like the odds or don't have the cash. Seems like players wanting to have everything their own way! If I were a bidder it would depend on the way I planned things when I was buying the calcutta.
Hu