I created a spreadsheet to calculate the cost of the "Dollar Colonia" with my bank's fees a couple years ago, but I need to update it. The last time I checked the numbers, it cost 9 pesos/dollar to go to Colonia for the day if you brought back US$5000. (I included all costs, including taxis, the boat to Colonia, and all bank fees.)
But there's a nice section to visit in old Colonia with lots of nice restaurants, if you want to make a weekend of it. If there's interest, we can create a thread to discuss the various details, options, and alternatives.