You can earn enough money to support a comfortable lifestyle. You might be able to swing wealthy with the right conversation with your DM.
Basically the way that downtime activities work in 5e is as described on pg 187 of the PHB. What it boils down to is as follows:
- practice profession (unaffiliated) - Modest lifestyle
- practice profession (affiliated) - comfortable
- practice perform skill - wealthy
So what you'd want to argue is that using your spell casting as a service in a shop would at least be equivalent to an affiliated profession and thus net you enough for a comfortable lifestyle. If you really wanted to push it, you might be able to talk your DM into a wealthy lifestyle.
If you don't want to settle for the professional abstraction, you might take your cues from page 159, the spell casting services section. This indicates that a first or second level spell may be cast by an NPC caster from between 10-50 gp.
Here's the thing with that though, what the lifestyle/profession numbers capture that the spell casting services don't (and why I'll use lifestyle/profession in my games) is the economic forces at play. YOu don't know how often someone in the general populace is going to need a first or second level spell cast, and thus you don't know when you'll earn gold and when you won't. This puts a lot on the DM to determine when it is otherwise properly abstracted by the lifestyle mechanic.