Opinions will vary all over the place. Room size will change the number slightly, but I'll bite.
Right around $10k
For that price you can get 3-4 VERY good subs and space them around your room optimally to avoid bass nulls. You can go to a Thigpen Rotary if you really want the subterrainian stuff for another, what, $25k (including everything you need [potentially room mods]), but I would say that 3-4 Seaton Submersives, or JL f113's or a few others should satisfy anybody for high quality and high output - you just need a way to control it all.
If you want to go DIY and are a fan of IB (and have the ability to install it in/around your room0, you can do it for less.
Obviously you can spend more, but that would be enough for me.
