Q:
If you search Alexa and see their ranking is good or if they are just starting out, which are usually the low spenders?
A:
There are so many answers to this question, it's hard to narrow it down. First off, MFA advertisers will spend low in an attempt to make money on Adsense arbitrage, and there's no predicting which keywords they will target.
Generally you will get low payouts on topics which tend to attract non-spenders such as kids, which is why entertainment and games won't generally pay well. You'll see a similar effect for low-cost items.
you might find this tool useful
http://uv.bidtool.overture.com/d/search/tools/bidtool/