How much money should be put away for a cat's lifetime?

Wow, that's a Very responsible way to approach this.

So, an inside cat (which is safer) can live on average 15 to 20+ years

Plan on a vet visit every year for shots- find out how much your vet charges for the visit, then add on average, another $50 to $100 for the shots

Emergency vet visits for an inside cat aren't that common, but having a thousand dollars or so on hand is a great idea (most emergencies will be less than this, but a serious illness could be more)

Food cost depends on what brand you go with and how big a cat you have (a large cat will eat more than a small one)

A good quality scratching post is something you should only have to buy once- they're pretty sturdy. The price can go from $50 (pretty small) to $1,000+. If you shop sales or find one on clearance you can get a good sized one for $100 to $200

If you start with $10,000 and add to it as you can, I think you'll be prepared for anything