I can add that I've been doing part-time tech support (only two weeks left! Hurrah!) for a while, and I'd say that of the Mac users that call in, most of them are still running Tiger. Leopard wasn't quite compelling enough for many people to justify the price tag.

That said, if you really can't advance your current version while remaining backwards-compatible, then why not fork the product and make the Tiger version a little bit cheaper? I've seen that from other developers, and absolutely love them for it.