Going from a wet sump system with crank case relief to a dry sump system picks up my race motors an average 25HP. The lowest gain that I have EVER seen back to back on ANY motor is 21HP. Again, this is just comparison between wet and dry sumps with aftermarket oil pans. A good oil pan, over stock, will net you around 20HP.
Going to a dry sump system, you can control the amount of oil pressure you want to run, also.
Moroso makes belt driven pumps that run $300-$550 depending on the number of vanes.