I've got a Sig P226 that alternates between .357sig and 40s&w with nothing more than a barrel change, since the .357 uses a .40 case. That's about as easy as conversions come (and barrels can be found for $150, give or take), but this is because one caliber isn't much of a departure from the other. For the same gun I've also got a magazine and top end to shoot 22lr.
All said, I've got about $1000 into a Sig that shoots three calibers. Not too bad.