i think 25 cent for 1k is not fair. I have seen other ads which pay u after 1.5 or 2$ for 1k impression.
Or in other words: Use a PPI and loose all your regular visitors And if you're really lucky and it gets looked at as mallware you'll get blacklisted on Google
cpm is intersting when you have lots of unique visitors. 40k impression is good, but how many uniques do you have?
Do you own a .COM domain or a domain with good traffic. Then Sedo is the place for you. Sedo is a free service that let you earn more money through their domain parking.