umm, higher is better. You don't get to choose it, it's based on how much you are earning - an extended trend estimate, I guess you could say. It's not unreachable to average over $80 eCPM with organic traffic and well placed ads (within TOS still, of course) For most niches, though, an eCPM of $5-$15 should be your goal, at least. Anything lower than $5 and you should definitely be trying to optimize things better, unless your site is really huge with organic traffic or in an extremely low paying niche.