Second best bang for the buck
Walk across the border, pass the first sets of cabs and walk across the street. (You will see a McDonald's.) Ask one of the Taxi Libres to take you to Adelita's or Hong Kong for $5 including the tip. If he tells you that Adelita's or Hong Kong is closed, get out and get in another cab. It's less than a 5 minute cab ride. (If it is daylight and you want to save the $5, keep walking along the pedestrian promenade to downturn, "El Centro". Just keep walking over the bridge and toward the big arch. When you reach the arch, walk one more block straight ahead, turn right and you are in la Zona. The walk is about 10 to 12 minutes from the McDonald's. Walk around the block and you will find Adelita's and Hong Kong and several other bars as well as the street girls that ScatMan's discussed.)
Enter Adelita's or Hong Kong. Check out the chica's and follow ScatMan's above dialogue, this time also asking for "oral sin condom." Tell her that you are willing to pay $60 for 30 minutes. ($50 if you are a hard bargainer.) If she demands $70 or $80, thank her and keep looking. Try to keep moving so that the meseros don't get you to buy the chica a ficha drink plus whatever else they can sell you. If you are familiar with the U.S. phrase GFE (Girl Friend Experience), the bar girl (BG) experience has the opportunity to be more along the lines of this. The street girl (SG) experience is often referred to as the DFE (Dead Fish Experience). However, some BGs are often only willing to give you the IQE (Ice Queen Experience). The oft-quoted phrase is YMMV (Your Mileage May Vary).
If you want to save a few dollars, convert your dollars into pesos before you cross the border. (San Ysidro Boulevard and Border Village Road usually has a good exchange rate.) Use 500 or 600 pesos as your offer.
Be sure to report back how it all turned out.
Happy Hunting!