This is akin to a question you've probably been asked.. "How much money can you make with real estate?"
You could make nothing or millions, it all depends on so many different factors.
Generally, generic .coms get some google love though there metrics are always changing, with this in mind it can be an advantage in ranking near the top with that generic .com but not always a guarantee, especially if it has no links, age, or is already a very competitive market with many big-name sites already comfortably in the top spots.. if this is the case, even with something like furniture .com (because of how competitive it is), you will still probably need to set aside a huge budget for seo and work hard to get it to that top spot.. even though the name is one of envy in the furniture world. A premium name like that if it receives no type-in traffic or has no links already even if it is great might struggle to make hardly anything without the proper guidance. Every name is its own opportunity though, and the possibilities are as ranging as those in the brick n' mortar world too, much of the same applies. You could pay a premium for some prime downtown Dallas real estate which would already come equipped with a highly visible spot and some guaranteed passer-by customers, but without the proper management it wouldn't grow or succeed. The advantage with a premium dot com is that some also come with those passer-by customers in the form of type-in traffic... very few will sell a premium on this metric alone though.. so again with the premium price paid will need to be a plan for growth if you ever want to take it beyond a parking page domain.