As Jimmy said, streetwalkers are pretty much your only option on that kind of budget, unless there are other cheap avenues I've not discovered yet. Plenty at night to be found all along Sukhumvit Road between Asok and Nana BTS, as well as on Soi Nana (the north end) as already mentioned. Just make sure you don't go into any bars to find a punt, as a lady drink or two and one or two for yourself, plus bar fine, and fee for the girl will probably break 50 even in the beer bars.
Just make sure you haggle, and don't ask them what price, instead tell them what you're prepared to pay. If they want too much and wont budge, start walking; usually they'll come down straight away. I find 1000 ST and 1500 LT nearly always works fine, but you might even manage to get it cheaper. I've not tried. What's important though, is you tell them the price, don't ask them, and be prepared to stand your ground.
