I agree with "col", it really depends on the market. One more thing that might change the CPM is the number of google ads you have per page. I usually put 2 or 3 ads. I have built many adsense sites on a wide range of topics over the last 3 years... and I can say that $8 is well below average. See a random sample of url channels in my adsense account... *Note* I can't post live links cause I'm new invulgotech.com/CPM.gif (screen shot) The weakest CPM I have was $9.18 for a video game site (you can't see that on the screenshot because it was under it). You can assume that each url is for a different market. I hope this gives you a good idea of how your site and market is doing. You can greatly improve your adsense CPM by testing with 2 channels and finding the best ad format and ad position for your site. If anyone wants some quick suggestions on adsense optimizing, you can PM me your website.
It all depends upon how many impressions that CPM is based upon. If it's based upon a low number, it's not very good or at least not very accurate. If that is based upon a higher number, then the averages have a chance of meaning something, in which case it's a pretty decent number.
Does the 8CPM stay there all the time or this is just one day? If you CPM is solid that means it doesnt change i.e. today 2CPM next day 8 then its good. I have sites that start with 100CPM and down the line at the end of the day its back to 2 or 3.