As we have sort of adopted this for Solaris and Ajax I wanted to throw this out there as a house rule:
When casting any domain spell that uses a material component with a cost in Gold Pieces the cleric may instead sacrifice experience points at the same rate used for the creation of magic items.
I think this is a good rule for both game and meta reasons. First It reflects the special nature of the domain spells relation to the deity's power/area of influence. Second it makes the Devotee, who only gets these spells but gets to cast them numerous times, more workable.
If you don't want to make this a feature for clerics I suggest we include it when we revise the Devotee