The obvious, simple answer is, the same people who spend $200-$700 for a single fan of a new introduction, based solely on a (well-massaged) catalog picture which depicts the best bloom the plant has produced to date.
Other excuses... Ahem, I mean reasons.
The seedling is from two very expensive or hard-to-get parents, the union of which piques the interest and imagination of the bidder, or one parent of the seedling is a tetraploid conversion of the latest "breeder's darling" diploid, a conversion which may be hard to obtain and/or use.
Some people would rather pay the money and see bloom next year, and, unless the seedling turns out to be a total dog, be able to integrate some rare or advanced genetics into their lines. It's a gamble, because it's a single sample of the cross, but compare this to them paying $400 for the two parents and then possibly seeing their own seedlings bloom in two or three years. There have been some nice daylilies registered which were the result of one-seed crosses.
Plant importations to the EU have been severely restricted as of late, and this may be the only way for the people there to acquire newer genetic material.
Human tendencies such as compulsiveness, greed, competitiveness and impatience play a large role in the final price.
This summer I bloomed a handful of (moderately-priced) seedlings purchased from the Lily Auction last year, and was surprised at their overall quality. I hadn't done much serious collecting for a while, and since I had none of the parent plants, it turned out to be a pretty good deal.
