While teaching my promoting class the other night, someone posed a question regarding book tours. They asked how is it feasible to do a physical tour when the average author only sells 20 books per signing?
There are many answers. Some author don't do book signings for this very reason.
If you're doing a tour or a signing, you need to get the most bang for your buck. Set multiple signings in one day or weekend. Stop at other stores along your route to sign copies on hand and leave promotional materials such as bookmarks. Be sure to do as much promotion as possible by contacting the newspaper and radio for articles & interviews.
Physical book tours can be quite expensive, especially for the publisher who sends their author on a month long tour with a chauffer, hotels and plane tickets. However, authors can tour for just the price of gas if they are smart, with an occasional hotel thrown in now & then.
And remember - sometimes success is measured in terms of exposure, not just sales!