Any assistance with the following would be much appreciated.

This is what I would like to achieve: I have a list of codes in a select. If the user selects COST then I would like a text field to appear on the form for the user to add a number. If the user selects anything else, then the text field would remain hidden.

If this is not possible another way might be to always show the text field, but to disable input unless the user selects COST and then force them to add something to the test field before allowing them to press submit.

I'm still learing, so any assistance would be of use and welcome. Thanks in advance guys.