The first screen you get to when you log in should show you your predicted monthly cost and current. Failing that you can use the Aws cost explorer to guesstimate how much
edit:
You can also go through the cost and billing manager in Aws.
Edit edit!:
You want to use glacier which is cool, pun very much intended. Plug your estimated usage into cost explorer and you should be good to go.
How deep is the litter? You may need a bit more in there for her or try swapping it out for something more absorbant like clumping litter. Our 4 year old still scratches on the side of litter tray every so often, it's most likely just cleaning bits of their paws.
Unfortunately cat shit really does stink, have you changed diet recently? We were told not to change diet too often as that upsets them ymmv. Once she starts going out properly she'll most likely end up going outside instead so that'll be a releaf. Our other 4-ish year rescue is so used to going outside she never uses the litter tray, our other 4 year old only uses it to wee in only (most likely a habit picked up while going through some rough cystitis patches), cats are just weird!