Well, digital distribution is a whole different method of selling from retail. I believe publishers set the price, and Sony takes a percentage cut (though don't quote me on that).
Some publishers have the balance right on the UK Store (bravo to Sega on that one, where their prices for PSP games on PSN are in line with "new" prices on sites like Game, Play.com, Amazon and ShopTo), but games like Prinny are £32 on PSN, and between £18 and £25 on retail websites. What I want to know is if Sony take that big a cut? I mean, surely the manufacture, distribution, licensing and retailer markup on UMD games will cost them more than the licensing and store percentage on PSN, right? At the very least I'd expect price parity with what retailers are offering on release date. At the moment, it just looks like extortion of the highest degree, whether that be by the publishers or Sony, I don't know.
Obviously there are more factors to them choosing their pricing (I have a suspicious feeling that the retail stores are one of those factors), but if publishers want to make a success of their items on the PS Store, and if Sony want to make a success of their download-only system, then they need to start doing a little bit of research first and giving their online-only customers a better deal.
I know RRP is around £29.99, but the RRP just doesn't reflect real retail pricing on new PSP games these days, and I think that's a huge barrier to the PSP Go!'s appeal, especially considering the system itself costs more.