Know-How Exchange : Research/Metrics

I work for a company that provides loan to private and legal persons. We run regular campaigns to get some attention to our products and to make our potential customers aware of our existence.

We have decided that it is necessary to measure effectiveness of channels (print, radio, outdoor etc.) and publications/stations we use. To do that, we record customer-calls (where it came from?; why?; what loan type they talked about? where they heard from us? etc.) and monitor our sales. According to information we can calculate all sorts of ratios (relative to marketing expenses) but today I can’t take much out of them.

Do you have any good ideas of how to improve our calculations (what other variables should we take into account etc.). I would appreciate it if you would share your experiences in measuring marketing ROI in your field of activity. Do you have found some "working" equations that help you to evaluate the effectiveness of marketing activities? Or maybe you have a good tip of how to determine the optimum of marketing expenses?

All answers are welcome and fresh ideas needed. I am looking forward to your reply.

Sincerely,
K.K.