I can't tell you how much. You didn't say which model iPhone you own. The back glass on iPhone can ONLY be replaced on iPhone 12, 13 and 14 models. If your iPhone is an older model, you'll have to pay for an out of warranty replacement, which is about 60% of the cost of the phone when new. Take it to the Genius Bar and they'll tell you exactly how much it will cost.
Unfortunately, we can't help with your personal circumstance.