You know, I once toyed with the idea of domain symbols. A cleric with one of these symbols could prepare spells in his domain slots as if he had access to the domain associated with the symbol, but would have to use the domain symbol as the divine focus (or as an additional divine focus, if the spell does not require one) to cast the spell. Naturally, a cleric should only be able to use a domain symbol if his deity or philosophy grants access to that domain, or at the very least, is not incompatible with the domain. I'd price it similar to a spellbook with nine spells, one each of level 1 to 9, or 4,500 gp.
As an additional bonus, a divine spellcaster that does not have domain slots could use a domain symbol to prepare up to 1 spell per level of spells he has access to from that domain (again, assuming the domain is compatible with his deity or philosophy), but no more than 1 spell per level of spells he has access to can be used in this manner, even if he has more than one domain symbol.