I've seen the connection block and charging electronics (to prevent overcharging and allow auto switching between the two batteries for about $40-50. A separate box to put the battery in would probably run another $20-30. Plus, of course, the battery itself. There are lots of options for less than $100. There are a few very good standard batteries that are around $50-60. You don't have to buy a deep cycle battery, but it does make a lot of sense in this application.
I go with a major chain for batteries, because your chances of finding one if you have problems on a road trip go way up. Plus, your chances of pushing the problem to a store manager or higher are better, if they really try to jerk you around.