A buck a mile? Seriously? I wish! If I got a buck a mile I would get an extra $50-100 a day just doing shops in town.
My advice is be realistic. Figure out how much it cost you per mile for gas, wear on car, and anything else that you need to be paid for in order to complete the shop. Figure how much your time is worth per mile. Add this all up, and bid the least you will accept to do the job. Then move on to the next job offer. Don't wast your valuable work hours engaging in back-and-forth negotiations.
Set your price fairly and keep it consistent unless something causes your costs to increase, such as higher gas price, need for a motel, etc. Once they know your price and your quality of work, they will only bother you with jobs that are in your price range. Not everyone is able to do a route for the same expenses. A shopper driving a Prius could probably do a route for less than a shopper driving a Hummer.