You can make a trickle charger for $15.

Go to Rat Shack and buy a 12V 1 amp wall-wart - they also sell large alligator clips. Get a pair, one red, one black. Also buy a cheap light timer.

Cut off the connector on the wallwart and strip the wires. Plug it in. Take a digital voltmeter and measure the DC voltage - check polarity of the wires and note which is + and which is - (by white stripe or by marking with white out).

Solder the alligators (red to +, black to -) to the appropriate wires. Set the light timer to stay on for a few hours/day. Plug the wallwart into the timer, connect the red and black to the + and - on the battery, and let it rip.

The 1amp charge will keep the batt in good shape, but a few hours a day will not overcook it.