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.
Otherwise you will have to send the relational values and store them in a Javascript array making them accessible at the Client/Browser side, when the onChange for the Cbox is triggered.
I can lend you some code to do it, if you are stuck in syntatic overload.