It's my impression that a this time there is no premium for rarity for the SP101 in .32mag. If anything, they seem to be selling for a bit less than the more popular .357mags. The 327 has sort of usurped it.
There are few SP101 collectors at this time so most of the buyers are shooters.
So -- I'd pay about what I'd pay for a .357mag in the same condition.
I have both barrel lengths and the shorter one is my wife's favorite piece. Oh, and I have only 11 SP101s.
Terry T