i tried to get a new slide for my uspc40ss to make it a uspc9mm. the slide is around 450 without internals? (extractor, firing pin etc.) all in all it's like 600 (including or not including the barrel). i would suggest buying another gun that's what i did and all i know is spent very little more but now i have 2 operational pistols vs 1 convertible one.
call HK parts and get a quote but these are the numbers that i seem to recall getting. but this is a little hazy and that was over a year ago.