Bitch tax is what you pay after breaking up with someone. It refers to the value of whatever personal effects of yours they had and now will not return.
John: She has my favorite shirt, three of my CDs, and my dog, and she's refusing to give them back now that we've broken up.
Adam: That's a tough bitch tax to pay, man.
Sally: I left my shampoo, makeup, and underwear at his place. Should I call him to try to get it back?
Jane: Just pay the bitch tax and move on.
Tom: Amanda stopped by to return my apartment key, autographed baseball, and the $30 she owed me.
Kristen: Wow, you didn't have to pay any bitch tax? Nice breakup!