If there's one marketing sin that people commit over and over again, it's that they try to spend too little money in an attempt to make a lot of it. The mailing list only thing you should focus on is ROI: return on investment. It's dollars spent versus dollars made. Nothing else matters. In this case, you have to generate the largest number of the most highly qualified prospects you can find, knowing that this will profit you most in the long run.
Once you've got a qualified prospect, you have three main strategies: you can upsell them, down-sell them, or cross-sell them. Upselling mailing list is just making them a bigger offer, or giving them more of what they bought from you the first time. If you plan it right, you can try to sell them a smaller package that makes it easier for them to make an initial buying decision, and then go for the upsell.
Upselling has probably been done on you many times, so now you have to get on the other side of the cash register and start mailing list doing it to your prospects and customers. Or you can down-sell. You can go for a higher-priced package first, using direct mail; and then you can sell a lower-priced package. As you can imagine, direct mail marketers don't do much of that.