Walmart offers the Schumacher XM1-5 Charger / Maintainer for $14.97. Opt for Site-to-Store pickup to avoid the $4.97 for shipping. That's the best price we've seen for a maintainer and the lowest total price we could find, outside the mention below. This device lets you charge your car battery from inside the vehicle.
Don't feel like picking it up? Amazon offers it for the same price with free shipping for Prime members.