Yes. We use "for" when talking about the price of something.
Can I buy this car?
Yes, you can buy it for five thousand pounds.
What's your best price for this Ming vase?
I can let you have it for two million dollars.
When you top up your mobile account, you're not buying a product. You're transferring an amount of money from your bank account to your mobile phone account. We "top up" those kinds of accounts "with" a specific amount of money.