10-14 bp A's are 1-2 ep (depending on skill), 15-20 are 2-3 ep (more depending on skill), 20+ can vary depending on rarity, skill, and market demand. For every +4-5 skill levels on a creature is worth 1-2 ep depending on the creature.
Skills are usually 1-2ep for a stack, except the downs, health, and revives can go from 6-10ep for a (4) stack.
16 bp AA's are 5-6 ep, 17-20 bp AA's can be 20-40, 20-26 bp can be 30-60, 27-32 can be 60-120, 33+ can be 150+ and these are basic guesses for reg lvl 1 skill 1 AA's.
These prices will also be dropping because of the huge number of AA's that have been hitting the market lately, so these numbers may be off by quite a bit.