Sometimes it helps to increase your price, actually. Because if something that you're selling is perceived as too cheap, people may think it's no good. Mail out a letter with one price. Then mail out a letter with a different price or a different payment plan. Compare the response.
Testing is a little tedious and laborious.
But it really works. You just have to keep track of each different variable you test. Only test one variable at a time. That means you can test one headline against another. Or one offer against another.