It wasn't that long ago that I remember being at a certain console Development conference and seeing a demonstration of the Havok physics engine, and being rather impressed with the capabilities of the engine itself, but then scared as all hell when they said how much they wanted you to pay in licence fees. Now, I'm not sure what the situation is with either Havok, or Math engine (which has now turned into Renderware physics, I think), with regards to pricing (as neither of them put their prices on their sites as far as I can tell), but we recently came across another physics engine called Novodex, that actually seems to sell for a sane price.
It seems to have a fairly decent API and be pretty stable too which after having doing a wee bit of physics stuff myself, I can say is the #1 most important thing for any game physics engine. We did notice a couple of problems with the amount of friction in the demos seeming like they were a bit too low, but all in all it seems like its a fairly usable package. Seems like it might be worth checking out if you want some physics middleware but don't fancy the crazy prices of the big boys.