This is so fucked up. He's a quote for you: "It's not your job to parent your parent."

See... this makes me feel like you're being manipulated and its working. Your mom knew that bringing this up would prompt you to make an offer. Then she took your offer and demanded more. Give an inch take a mile. $500? I feel like unless you're a doctor or lawyer, that's a significant portion of your paycheque. Especially over time... that's a lot of money. Probably looks similar to child support... except it's mom support. :-/

Here's a life hint: She's not entitled to your money. Presumably, you're in America. If you want money, you get a job. I mean, unless I'm missing key data here, she's able bodied right?

Here's another future lesson you can avoid making for the first time, every time that you'll giver her a huge sum of money, you're going to feel like a dumbass and lament that you couldn't put it towards ANYTHING else. I'd rather see you spend it on pogs and penny candy.