Based on what? In this post-covid market, a 20 year old beater can fetch $1200+ unless it's located out in the boonedocks or salvage title. I'm sure some auction prices are lower, but that's a gamble in itself and vehicles likely needing work/$$$$ put into them.
I haven't priced Volt batteries recently but would guess that the battery alone is worth near twice that, unless it has very high miles.
I've been looking at Volts...
Most 1st gen Volts I see are being offered from around $6k to $12k. 2015 is the newest of the 1st gen, so the higher figure seems more correct.
I've seen several people on the gm-volt forum talking about dealers offering around $1500 for their Volts if they are having problems that suggest battery issues. Some owners have sold newer 1st gens with battery problems for as much as $4000 to peddle.com, but I think those were pandemic pricing.
Few, if any 1st gen Volts are under battery warranty. A "rebuilt" battery from Greentec is $6000 or 10,000 with an 18-month or 36-month warranty. New batteries are not available. I'm not sure if GM is even selling batteries currently, but I believe they were also rebuilt.
It's often said that Volt batteries are expected to fail at around 10-15 years of age. Also, they aren't quite as easy to work on as Prius batteries.
Despite all this, I'm still thinking about getting one! I'm blessed to have skills and a place to work on cars, so I wouldn't be afraid to dig into one and try to fix its issues if it started to act up. And in the worst case, if I had to walk away from a "bricked" Volt, the loss would sting, but we would survive. For the OP, though, this sounds like it could potentially be a nightmare.
I am assuming the OP is getting the used electric vehicle tax credit of 30% of the purchase price (note it is a nonrefundable tax credit). Might even be available on the Leaf.