The ad does not mention price, but tells the reader to write for free details. Once you have an inquiry, you then send information concerning what you have to offer, and watch those orders pile in. Essentially, a mail order classified ad buys a name and potential business. It may just break even with the actual orders placed. The real money comes from subsequent sales.
Profit begins at the second sale and gets bigger and bigger as the sales continue.
You'll get far more responses from an ad for free information than you will for gods - at any price. Charging a small fee to cover postage or the cost of the inquiry will never make you break even - people won't be bothered. And a small fee doesn't necessarily weed out non-buyers. If you find your classified is pulling people who aren't interested in your offer, you can change the advertisement to be more specific as to what you really have for sale. The best advice in placing classifieds is to follow the leader.