There are IP camera 12V UPSes you can get that are ok -- I have a few TalentCell 98Wh units -- but they don't actually deliver regulated power, they just tap directly off of the li-ion cells. Those are about $65.
With an inverter UPS, $50 gets you a unit with a single 12V / 8ah or so lead-acid cell; 90 or so watt-hours _could_ be reasonable except that the inverter draws upwards of 10-15W while running, so these units actually won't run even a router for more than a couple of hours. Also, lead-acid dies after about 3 years, and then you're in for another $15 or and a bit of a pain replacing it. I'm really sick of doing surgery on old UPSes at this point and trying to move entirely to LFP for the 10 year lifespan alone.
If you want really long runtime, you basically have to DIY, but it's fortunately quite straightforward. Larger LFP batteries are about $250 per kWh, sometimes less. DIY'ing it, you can pick your capacity/runtime independent of your wattage, vs pre-packaged UPSes that frequently couple the two.
(I have a big pile of APC UPSes of various sorts, some with external expansion battery packs, and it's quite a hassle to get truly extended runtime from any of them -- I don't want to have a generator, I want 20 hours of extended runtime. I realize I'm a little weird, but I like to hope I'm just ahead of the curve a little on switching. :-)
