12v battery
$500---yikes! I just looked into this battery topic the other day. Exide/Champion make a lawn & garden battery (the GT series, price range $30-40) that should fit the Prius battery compartment (the OE battery measures 5"W x 8"L x 8"D). The problem is the terminals, which on the OEM are one centimeter (3/8"+) in diameter: considerably smaller than in the standard American automotive battery. Plus the terminals on the Exide battery are not your standard automotive either, but "L" shaped with a hole in the vertical part of the "L". With a couple of sets of 3/8" nuts/bolts/washers from the hardware store, you could probably use the Prius battery cables with the "L" terminals.
The top of the line Exide GT series is rated at 380 CCA (300 CCA @ 0 F.), which should be plenty enough to nudge the hybrid system into ready state.
Exide batteries are carried at your local KMart if it's still open.