It would probably be frowned upon to take the hotel's towels out of the hotel (especially because towels are one of the biggest "walk off" items in a hotel & a major expense to replace constantly.)
That said, if you do decide to take your hotel's towels, just be sure to bring them back. (I'm with you...$4.50 is ridiculous to rent a towel, when I could buy one for that price at a shop!)