I am going to push the point and ask what are we going to do with the hammers? Why not have Commerce run scientist instead of building levee? While it will generate more hammers than it costs, I am not sure those hammers will out produce sci specialists in beakers.
Commerce will grow from current pop of 15 to 26 (assuming 15 fpt from sushi and working all cottage tiles)
It currently has 1 spare healthy face, so we need ~10 more.
It will have 26 total unhappy faces (from crowding), and we have 22 happiness atm, so we need 4 more happy faces. (We will get some wonders, but some things like Ivory will become obsolete).
Therefore, it needs:
Theatre (50h), 1 happy
Colosseum (80h), 1 happy
Grocer (150h), 4 health, not to mention +25% gpt in our biggest commerce city
Hospital (200h), 3 health
Confused Temple (80h), 1 happy
Laboratory (250h), +25% bpt in our biggest research city
Supermarket (150), 4 health
Levee costs 180h to build, which is ~10 turns working both mines. Working 2 scientists during this period will produce 2 * 6 * 235% * 10t = 400 beakers.
Assuming we never work the mines again, and work every cottage available, the levee will boost its base hammers from 6hpt to 22hpt.
The list is 960 hammers, and these don't even include 'nice-to-have' items, they are all essential to maximise its population and output.
The laboratory alone makes the levee worth building.
edit: by the time we are building the laboratory, we will have ~10 scientists, and most if not all of the cottages will be towns, therefore we will have ~200 raw commerce a turn. The 25% bonus from the lab will be 50bpt, so if building the levee speeds up the lab by 8 or more turns, is is beaker positive.
The Lab will take 42 turns to build a 6hpt, or 12 at 22hpt. = 30 turns. Sure we could whip it, but that will cost us specialists, so its much the same equation