IS it the battery or something else?

What's going on with my van. It's been fine until this morning and all of the sudden and the ac and the radio were turning on but the car just wouldn't start and the speedomoter was going from 0-120 and then back to 0 and so did the "cold and hot needle" and then all of the sudden the radio turned off but the ac was still working. I did not leave any doors open, any lights on, and anything on at all so I have no idea why it is doing this it's at 2003 dodge caravan.

Should we just jump the battery after my husband get's home and see if that will hold or just buy a new battery? How much do batteries cost for this van?