For the most part, you get out what you put into it.
It also depends on your business model.
For eCommerce stores, email marketing (if done well) can "easily" double sales ... pretty much, automatically.
There are upsell emails to get people to buy more. Cart abandonment emails that remind people to finish their order. Promotional emails that drive sales. And more. It's insane how much email marketing can increase an eCommerce stores revenue.
For B2B, it may be a little different; however, the customers may be bigger. What if one customer was worth $10k/mo? And if your email marketing efforts only resulted in 1 new customer a month ... would it still be worth doing?
Let's take it down a notch. Maybe you only sell $50 products, and by implementing email marketing, you're able to sell 10% more a month. Would it be worth doing?
Some other benefits that increase the value of email marketing are that it's fairly inexpensive, can be highly automated and hands off, and it's easy to get started.
Plus, there's the "value" of being able to converse, 1 on 1, with your leads and customers. Having direct communications with these individuals is invaluable to a business.
Needless to say, it's very hard to place a distinct "value" on email marketing efforts in general. There needs to be some context; however, 99.99% of the time, the value of email marketing will far outweigh the cost that goes into implementing it.