You need the textbox to change its value depending on a combo box selection. This must mean there is an onto mapping between Cbox values and Tbox values. Further it seems as if this mapping resides on the server side, as I can gather from your description of the problem.
In this scenerio the easiest way is, to do a quick Ajax call to the server with the new Cbox value when it changed, and fill the Tbox with its corresponding value on the Ajax reply.
I can lend you some code to do it, if you are stuck in syntatic overload.