I always carry pesos, girls have quoted me $50 dollars and I'll hand them $500 pesos and they'll accept it - if she doesn't, cut her loose and shout "next"
. At the current exchange rate, that's a $13 dollar difference. I hate to generalize, but most Mexican girls are terrible at math anyhow, so they'll feel more comfortable taking pesos or multiplying by 10.
Another technique that I've used in negotiation when the girl's quote was higher than market value, put your cash out on the table. This will A) embarrass them because everyone in the bar can see she's getting paid for sex B) talking about an abstract $ amount and seeing your money on the table are two different things C) DO NOT PAY them until they are in the room with you. (except at strip clubs or bars with an exit fine) D) make all agreements before you leave the bar so there are no misunderstandings in the hotel room. Most of these girls are not professional hookers per se, they may not want to give you a blow job, "all night" might mean til 2 AM, bringing 2 girls might not be as fun as you hoped as the girls won't touch each other. These are all things that are better discovered in the bar rather than your hotel room.
One of my regular go-to girls was in bed with me and I said can I CIM? She says, Oh baby, you know I don't like to do that. I said I didn't ask if you liked it, I asked if I could do it. She nodded yes.
Here's another tip when you hit the Casa de Cambio at the bridge, if you exchange $100 or more ask them if they'll give you a better exhange rate if you exchange $X - if they say no, tell them you are going to another Casa de Cambio (there are at least 8 near the bridge) - I routinely do this and get 5-8 pesos per hundred dollars more than the exchange rate on the sign. That may not sound like much, but it all adds up, right?