As the bid cost goes up, so does the CTR, but does the conversion rate keep up? Also, the risk increases, since th cost per click is higher. In my world higher risk means the profits must be more than just proportionally higher... 10% more risk needs about 30% more profit, whereas 50% more risk needs like 200% more profit, etc... since the risk involves off-site factors like domain management, backlink management, etc.
I have found in my most of my areas that bids over $.15 or so require conversions over 35% to be worthwhile, but this will vary with your industry/niche/definition of conversion/profit per conversion/overhead/etc.
I have one area where $1 bids are worthwhile at 10% conversion, but that one is unique for me - it may be typical of high value conversions, but in this case the risk associated with the keywords is very low (as long as it stays under the radar!)
Definitely. Affiliate programs from MYAP (for example) have subIDs. Use those to track the conversion rate of each keyword/adgroup. Set the bid price based on the invidividual keyword/adgroup conversion rate.