Think of it the other way around: how much per month is Google making by reading your email (with bots) to generate a marketing profile so companies can buy targeted ads to sell you stuff? Besides, feature-wise, you get a LOT more from ProtonMail than, say, Fastmail, which is the low-feature, no-frills, non-encrypted alternative.
If you aren’t paying for the product then you ARE the product.
If you aren’t paying for the product then you ARE the product.