There is really no limit when it comes to how much a newsletter owner can earn when publishing email ads.
The potential for your ad earnings really depends on three things:
(1) How large your email newsletter list is - the more subscribers, the bigger the potential there is for you to earn
(2) What your click through rate (CTR) is - if you have well engaged readers who consistently open and click on links in your newsletter, your earnings will be higher
(3) Where you place the ad in your newsletter - ads placed at the top (in the viewable content on first open) will perform better than ads in the footer of the newsletter
A quick way to calculate estimated earnings is: [total subscribers] * [CTR] * [average CPC].
For example, a newsletter with 10k subscribers, a 5% click rate and an average CPC of $1.50, will earn: 10,000 * 0.05 * $1.50 = $750.