Not enough info; clarification needed. Is your front a double or triple? If double, you'll get the most bang for your buck by upgrading to a triple. You can get low end MTB take-offs pretty cheap (<$30, with chainrings) on Ebay all day long. If you want something nicer, well, you're going to have to pay a little more.
Judicious selection of chainring tooth counts will yield you a wide range with close ratios and minimal duplication.