Hey Brian,
When I first got into pre-war a few years ago, I used T206.org (thanks Trae) as a "guide" to help me when I was ready to pull the trigger on some T206's. Right now, it seems like 206.org is a little low on their valuations and just needs updated. You are dead on by looking at Ebay completed listings to find the price...but know that they do vary depending on a lot of things: grading company, appeal (big one) and how they are advertised. These are just a few big reasons they sell for the prices they do on Ebay...there are other reasons as well. Check out vintagecardprices.com sometime...that's a good place to get price estimates on cards as well.
Here are some price range for commons:
Poor to Fair - $12-$18
Good $20-$30
Very Good $30-$45
VG-EX $45-$60
EX $85-$110
Remember that appeal has a lot to do with the price set...not always the technical grade!
One great thing about T206.org is that it is a great guide to look up and find different variations, which are worth more...maybe a lot, maybe slightly. One example is Ed Reulbach, "Glove Showing"...this version is priced higher than "no glove showing"...this is where T206.org helped me learn more about the different values of commons.
With trimmed cards, the prices dip quite significantly. I'd say most trimmed T206's sell as if that card graded out as if it were in "Poor to Good" condition, unless the card is very rare or something of that nature. The prices do vary, but usually the selling price is close to the poor to good selling value of that particular card.
With any card that has paper loss, the highest grade it should receive no matter how nice it is should be a Good "2", but if the card is in outstanding condition otherwise, the selling price should bring in more because the card has better appeal than most "2's". However, I'd say cards with paper loss are graded as Poor or Fair in most cases...I was just saying that cards with paper loss should grade no higher than a 2....but unfortunately there are a lot of cards out there in high graded slabs with paper loss deceiving a lot of people, but that's another story
Sorry for the long rant, but I gave you all I had. Good luck collecting!