6000km sounds like a lot, but the Chinese have built a 3000km UHVDC line delivering 12 GW, and putting down submarine communications cables this long is complete routine today. Would be interesting how much aluminium/lead/copper such a project would take. EDIT: found a supplier that specifies a 1GW cable at 7000 tons per spool. A spool is 130km of cable, so that's 350 000 tons of cable per GW for the transatlantic link. So just the raw aluminium is around a billion dollars per GW.
Anyway, first we have to properly connect those two sun belts to the rest of their own continental masses with UHVDC, then we have a lot of political problems to solve, and then we can check battery prices...
[1]: https://nato-l.com/ [2]: https://redefining-energy.com/