Other costs in no particular order and whether it is one time or on going.

Dirt work,  1

Rock,  once maybe twice plus a little gravel once in a while

Bring in electric, water and propane,    1

Pay for electric, water and propane,    on going.

Property taxes,    on going.

Property insurance,    on going.

Insurance on the livestock?   On going.    If you're custom feeding the owner of the livestock likely carries this but make sure you know.

Power washing,  on going.

Repairs, this won't be much initially but you can still lose ventilation controllers due to power surges and fan/feed motors will up and quit randomly.    On going.

Eventually, replace gates and feeders.   How many times and how often you do this depends on how long you use the building and who you feed for.

I'm sure I'm leaving some out also but that is what I can think of for now.

Cash flow?   Depends on how long you want to be paying for the building and how cheap you want to work.

If building a new building to custom feed in and trying to make it worth your hassle build at least a 2400 hd building and try to find a site suitable for building another building some day if you wish.   

You are married to the hogs (chores have to be done everyday) whether you have to do chores for 1200 hd or 4800 hd.   Driving to the building/s takes the same amount of time and it takes the same amount of time to shower.

Have you worked with hogs fulltime before?     If not, that may be a good idea before making a commitment to take care of hogs for the forseeable future.