Fast facts first:
1. Before a player can be signed to PS he must first be released during cut downs, at which point he is free to sign with somebody else without compensation.
2. Once he's on PS, anybody can sign him away without compensation provided they place him on their 53 man roster.
3. PS guys don't count against the cap.
Assuming you re-sign your pick after release, he'll typically be paid the PS minimum salary...$5,700 per week for each of the 17 weeks, or $96,900 for the year if he sticks for the entire season. A guy might get paid more if there's some competition for him as a PS player. If (foggy) memory serves, I believe we paid Shaky Smithson about double the PS minimum, though I couldn't begin to explain way. Every once in a while you'll hear of a PS guy getting paid around the rookie minimum, but this is pretty rare.
Getting to your example, lets say we drafted an OT in the 3rd. round (or higher?) because of Sherrod uncertainties, but Sherrod performs through pre-season like the second coming of Chad Clifton. MM wants to carry 7 O-Lineman. So, can we stash the 3rd. rounder on PS, even if we paid him a lot more than the PS minimum? Probably not. It's highly likely one of the other 31 teams would think enough of that high of a pick to find a spot on their 53 man for him.
Part of the reason we kept Boykin on the 53 man last year rather than putting him to PS might have been because they thought another team was sniffing around him.