You can usually expect to be charged 5 to 20 cents per deliverable message, and you should expect any e-mail addresses that mailing list are "bad" or that "bounce" to be replaced by addresses that are current. If you decide to use a service like this, it's absolutely critical that you find out how the e-mail addresses were obtained.
You want e-mail addresses that have been collected ethically and responsibly, and this means that you want the e-mail addresses of people who (a) are directly interested in your product, industry, or field of expertise, and (b) have mailing list given their permission and "opted-in" to the list. Important Note: If you buy lists of e-mail addresses that have been "harvested" from newsgroups, classified ad sites, online services, etc.,
you'll be accused of spamming! These people have NOT given you permission to contact them, and you can get into a LOT of trouble this way. Again, I can't overstate the importance of making sure that the e-mail addresses you rent have mailing list been collected ethically and responsibly! Buying or renting e-mail addresses this way can get very expensive, so you need to be sure that your entire sales process i.e., your web site.