"In some proposals, a line item may not be used, therefore it must hold a value of "NA"" - why?
Why not leave the number fields you have to do the job that numbers do, and have another field (like you have created already) called, for example, 'ChargingBasis'?
You could overlap the two type of fields, the charging basis and price, set the charging basis to be transparent when empty and the text color to be the same as background if Price = 0. you could also place the text "Hourly Basis" just to the right of the price field and have it conditionally formated as well to be the background color when the charges are not hourly, but the normal color when it is hourly.
Thanks for the replies. Mark, I went with your repsonse, and it worked great.