How much gas $$ am I supposed to pay a friend?

$5 I suppose. If you are too exact, down to the nearest cent, that may strike them as a bit odd.

Give your Friend about $20. do not forget, they're also taking time out of their day to assist you.

As someone who frequently has to get a ride, I find out it this way, How much gas could I be sure is in my own car to make the same trip? Then I reimburse them on that base. Or make a pre present reimbursement amount between you and your friend.