Some list owners will inflate their subscriber numbers to encourage advertisers to use their services. The first thing I do is subscribe to the newsletter to see the quality of content and what others are advertising via solo ads. Then I run a classified ad in the newsletter to test the responsiveness of the subscribers. This also gives me an indication of whether the stated size of the list is true or not. I have had better response from a list of 20,000 than what I received from a list of over 200,000.
Many factors can influence such a result.
While you should receive a far better response from solo ads than classified ads, it still pays to test wherever you can. If a classified ad in the newsletter did not result in responses, I would not advertise using the solo ads. However, I might test a few different classified ads to see if the initial results were correct before moving on to other possibilities. It is worthwhile to do the numbers before spending on advertising. While many marketing experts talk about the lifetime value of a customer concept, those whose business activities centre on promoting the products of affiliate programs should regard this with caution.