Don't pay for a downgrade guide, the information on how to downgrade is 100% public. Not every version can be downgraded yet, generally the newer it is the less likely it is to be downgradeable.
actually, unless you own a psp slim (which is capable of having a custom firmware loaded on it either way), you can currently downgrade all original psp's with use of the Pandora's Battery method developed by Team C+D (www.Noobz.eu).