None of the stuff has any intrinsic value – it’s worth exactly a) what you’d take from someone else for it or b) what someone else would pay you for it, whichever’s higher 🙂 . To convert a price listed in the various guides to cash, you have to find someone willing to pay you that amount. You won’t make a killing as record dealers generally pay you only about a fourth of the listed value.
Among KLF collectors some of the harder-to-find stuff is obviously going up in value e.g. ‘Space’ CD; but The KLF aren’t exactly Led Zep or Kate Bush in terms of having a widespread fan base, so in the larger market, you’re probably not going to have much luck making $50 off your “All You Need Is Love” 12″. In fact the price of the KLF releases is actually falling with time (or to be more accurate they are not selling at the higher price any more, yet record shops haven’t noticed this and reduced the price yet).