Such a loaded question...
By dealing the "pro-spot", who'd be YOUR 13th player you'd now be forced to cut? WHICH pick in the 15th round would you be receiving with the pick etc...?
My advise would be to go to established 24-team BL league websites (BL-DirtyDoubleDozen, BL-Mattingly examples) and scroll through years of their transactions pages to gauge value...
In most cases, Rd #13's (protection spot) are more valuable than even round #14's in 24-team BL leagues. Sure, there's always that fringe keeper from the previous season that's dropped & surprises. Arrietta in "15" & Hendricks in "16" are examples, but in my experience, it's slim pickings after the 1st 5 picks (roughly) are taken in the 14th round. You'll have 2-3 top international players entering the MLB & become available (Darvish, Abreau, Meada) 1-2 decent players which were dropped, then RP's and fill'ins...
Without players crossing leagues (AL to NL, NL to AL etc) there really isn't much talent available from the mid 14th round on, let alone anywhere in the 15th...
Just MY opinion...
-GW-