I do not know about Cartagena but I was in Rio last year and craigslist works down there.
I found a beautiful woman for $300. for 24 hours. I could have bargained for less but WTF.
I gave her $900. and we fucked like high schoolers for 3 days and I had someone for dinner/drinks and a tour guide for 3 days.
The dollar is really worth something down there, so HAVE FUN!!!!!!
Ii have been to Cartagena several times (ex wife is from there). It's awesome and safe. Do not stay at the Hilton, it is not guest friendly. Rent an apartment in Laguito right by the Hilton. Juan Ventura is the man to go to and he is one of the most straight up guys you will ever meet. ittscol@hotmail.com
You won't need any help finding the women. La Dolce Vita at night which is walking distance from the apartment you will be staying in and there will be plenty of guys to explain the deal.