I've been monetizing data for nearly 17 years, so I'm quite biased to it.
The really short explanation is...
I acquire fresh / targeted 3rd party email data, which is data that users have shown an interest in a specific niche and have provided permission to receive messages from third parties. I get the opt-in record of all users and it's 100% can-spam compliant.
Unlike most marketers always spending their money trying to get traffic, I spend it on data because it's an asset that I then own and I can market to the users over and over at low cost.
I get data that targets users that are interested in specific PPL (pay per lead...lead generation) offers because there is no credit card / purchase required to complete an offer. All a user has to do is fill out a form, so conversions are typically much higher compared to offers that require a sale to be made.
Some of the verticals (niches) I have done extremely well with are: insurance, education, debt. loans, credit, mortgage, assistance, discount offers, homeowner offers, etc...
The bulk of the PPL offers I promote pay $20-$40 per lead, but I promote others that pay more and less.
3rd party data is rarely as responsive as a high-quality opt-in list. So to compensate you have to work with higher volume. This isn't an issue because 3rd party data is much cheaper and highly scalable compared to building opt-in lists. The key to making it work is to always be collecting your opens / clickers, segmenting and removing unresponsive users. Essentially you are converting the data from quantity to quality.
Basically, I monetize the data using PPL offers because they offer the path of least resistance to generating conversions / $$$ and converting the data into cash producing assets.
Obviously, there is far more to it, but done right it can be extremely profitable. Everyone that I know that is in the business and knows what they are doing, for the most part, does 6-7 figures. While that is a huge range, much comes down to one's ability to scale ... effectively implement / manage the infrastructure needed to scale.
Anyway, something to think about.