I don't know if there is a "less expensive way" of converting PP to BTC. The problem is that PP payments can be reversed, and the PP sender usually gets the benefit of the doubt if PP has to make a judgement wether to reverse the payment of not.
This is one of the main reasons why you won't find a lot of exchangers willing to trade PP to BTC, and most of the ones that do will charge big fees (and usually won't accept trades from lower ranked members).

Your best chance would be to trade with somebody you know IRL... I wouldn't mind accepting PP from a real life friend in exchange for some BTC.

Other options would be sites like virwox.com, but their fees are also enormous.

Good luck