I just bought a .357 SIG drop-in barrel from EFK Fire Dragon (an Arizona Gun Runners associate) for less than $140 (Prices subject to change). Literally dropped in and was ready for use. I've only put about 100 rounds through it, though, since .357 SIG ammo doesn't run cheap.
I had no malfunctions, and my groups were as tight as in .40 S&W. But I do recommend a 19 or 21 lbs recoil spring for the .40 to .357 conversion, as the original 16 lbs factory part will not absorb the full brunt of the .357 SIG load, and using it frequently will unnecessarily wear out your gun, especially at the rails. I'd personally go for the 21 pounder. The slide might be a tad harder to rack, but the gun will still cycle, recoil less, follow-up easier, and live longer.
I get my gun springs at www.gunsprings.com