Honestly dude, I'm on the owners side. I've worked in retail before and can tell you're one of those customers who tries to haggle too much.
You also keep trying to get a different answer to the same questions (but your dad said this... blah blah). They're obviously not interested in bringing the prices down. I would be sour with you as well if I were them, answering questions (be it the same one) over and over again take up time, time that they should be investing on running their shop. Don't expect them to spend hours going over info on a $3 product.
If I were you, I'd try to get back on their good side and just be happy as they seem to have good prices.
Otherwise, just go to petsmart, you don't need to buy mice/rats from experts.