The alarm industry has used small 12VDC independent power supplies for a long time now. This model, the AD12612...
http://www.shieldlab.com/forum/index.php?action=search2
is more than sufficient to handle your pump and comes with leads to attach to your 12V battery. Markets at this site for $20, (And thanks for letting us know you're in the USA), but you might be able to find a better price by googling the model number.
It does require a 16.5VAC 40VA transformer, which is plugged into an outlet and everything from there is low-voltage wiring...
http://www.shieldlab.com/forum/index.php?action=search2 Markets here for ~$13, bringing the total price to ~$33,
But you can definitely find a better price on eBay, or Google-search using "16.5VAC 40VA transformer." For example...
http://www.ebay.com/sch/i.html?_odk...r.TRS1&_nkw=16.5VAC+40VA+transformer&_sacat=0
At the moment there's a $7 transformer available that with free shipping, bringing the total down to $27
If you want to be clever, there are old obsolete alarm control panels, still completely functional, that use the same transformer and have an Aux. 12VDC output of about 500-600mA off the PCB terminal strip.
http://www.ebay.com/itm/Honeywell-A...994705?hash=item3ab348c851:g:9G0AAOSwCypWpSr2
You could repurpose it to use just the power supply portion of the panel and save a few bucks. The only downside would be the bulkier PCB, but it would bring the total cost down to $21.
I used to do that all the time when I upgraded working alarm systems: Used the old panel, instead of charging customer for a new aux power supply when one was already on site.
EDIT: If low cost is a real consideration for you, I have a bunch of functional PCBs with 12VDC outputs, and transformers stashed away from my years as an alarm tech. Let me know, and we'll work something out. One caveat, I'm away from home and won't be back before the weekend at soonest.