I'm with Matt in that there isn't a correct answer, only what i am comfortable with.
I hunt hard and long not for a guide, but for someone who is both a guide and someone we will enjoy traveling with. If I choose well, then then we have someone to talk to about culture, norms, history, family, etc. ... and the sights. For that I buy an occasional lunch and at times even more. We took our Odessa guide sailing with us, the one in Montenegro we took fishing with us. As a result we pealed back another layer of the country and it added enjoyment to the trip. But Im pretty social when i travel and i just plain enjoy it. In return our guide in Bulgaria took us home to mom for home cooking, the one in Kiev brought a gift basket each morning. We stay in touch with most of them and have used a few multiple times. ON the other hand, the last guide we had in Lviv was "efficient" but a bit politically warped and so she got nothing.
Generally for slightly better than normal we tip maybe 10%. For a quick one hour guide through a church or the like, maybe 5 euro.