Way 1 · best default
It's a bit out of my budget
Polite and honest. The problem is your budget, not the price, so nobody is offended.
Examples
It's lovely, but it's a bit out of my budget.
That hotel's a bit out of my budget. Anything cheaper nearby?
In a conversation
A: This one's our best model. It's 900 pounds.
B: It's a bit out of my budget. Do you have anything around 500?
Common mistake
Not: It's so expensive for me.
Say: It's a bit out of my budget.
"Expensive for me" sounds like a translation. Talking about your budget is the natural, polite version.