Hi,
There's no real price guide for spells, prices vary by the seller. Here's a guide for crafting the spells:
http://www.crys.org/everquest/spells/researchnec.asp
I'd say a fair price is equal to the level of the spell (level 20 spell is 20p, lvl 34 spell is 34p and so on), with the exception of the level 49 spells which are typically 50-100p each.