Dismantler/eBay pricing (as usual, your mileage may vary and this is just the cost of the engine alone):
2.5 base = $3K and up
2.7 base = $4k and up
3.2 S = $5K and up
The cheapest engines will likely have 80,000-120,000 miles and not have any upgrades (RMS, IMS, oil pump, water pump, alternator, etc). Add $2K-$3K for an upgraded engine. Add another $2K for a lower mileage engine. Add another $5K-10K for a professionally rebuilt engine with all the upgrades.
986 roller (no engine) is worth $3K-$7K depending on year, model, mileage, options, and condition. Expect lower than higher.
Wayne has an excellent article on engine swaps here:
Pelican Technical Article: Boxster Engine Conversion Project - 986 Boxster (1997-04) - 987 Boxster (2005-08)