$10 for one https://www.amazon.com/Automatic-Battery-Charger-Maintainer-Motorcycle/dp/B014QXL438
Or two for $18 https://www.amazon.com/gp/product/B082X75Z3M
The world has gone mad for LiPos but many rolling robots (Arlo, HERO, etc) can still make good use of old school lead acid cells. I paid $12 a few months ago for this small 1.2A automotive chargers for my Corvair while I'm working on it. Works great to charge and float at ~14.4V, better than those HF float chargers which won't charge a low battery. It also charges SLA batteries just fine, potentially useful for mobile robots. I added the $1 DVM display in the photo. It comes with a simple LED smiley face, red=charging, green=floating. Good enough for ordinary people.
Now Amazon offers it for $10 or a 2-pack for $18. I'm trying to resist buying more but it's a great deal and this hoarder's mouse finger is getting itchy...