One solution would be to use the VLOOKUP function.
Lets say you have the text values a, b, c, d, e, f in Cells A1:A6 and this is the list that you're using for your Data Validation (let's say that is in E1). Now let's say you have values 1, 2, 3, 4, 5, 6 in Cells B1:B6 and these are the corresponding values for text in A1:A6. And you want the person who's selecting a text value in E1 to have the numeric value appear in F1. The following formula in F1 will do this: