If you are the seller of sex you want the price high.
If you are the buyer of sex you want the price low.
If you are the buyer of sex and you can get it cheap you are considered lucky.
If you are the seller of sex and you give it away for free when you could get a better deal, then you are STUPID SLUT.
All sluts are stupid, so "stupid slut" is a PLEONASM like "tuna fish".