In the portable storage device market, microSD cards tend to be cheaper than Pro Duo sticks. Currently, there are adaptors already out there that make microSD compatible with the PSP's Pro Duo memory card slot but not the high capacity microSD variety. However, PhotoFast has just announced one such device that is compatible with both microSD and microSDHC cards up to 8GB.
The PhotoFast adaptor (known as the CR-5300) is compatible with both the original PSP and PSP-2000 (as well as with other Sony cameras and mobiles using Pro Duo) and is expected to cost around the £12 mark when released shortly.