| Matthew Jacoby |
It would depend, to me, on where exactly the project intends to go with this.
What I see as d20 Modern's primary advantage over D&D 3.5, and especially D&D 4e, its its design towards genre flexibility. With minor expansion books, it was able to go from urban fantasy to historical fiction to horror to sci-fi smoothly, or at least as smoothly as the 3.0 edition based rules were able to manage for anything.
I really enjoy what Paizo has done with 3.5e D&D. Paizo's work is of such quality that I'm certain I would pay $10 or $20 for a pdf of such a game, no matter what you decided to do with it, because I feel that it would probably be a pretty good game.
But I think I'd only want to put $50 down to get a hardcover copy if it was a strong objective to retain that flexibility. There are a whole lot of RPG systems out there, covering individual genres decently, and much fewer that are built as flexibly. The only one I'm personally aware of is GURPS, which in my experience is nightmarishly more complex and time-consumptive to prepare for.
That said, I've been waiting a long time for a decent successor to d20 Modern, and would happily pay more than $50 for such a game if it included sci-fi and fantasy expansions, or a simple enough scheme of compatibility to use old d20 modern or other systems to smoothly add high-tech and magic.