Hey Matt!
Typically, a single 28mm figure could take about 5-15 hours to sculpt, depending on the type of figure it is and the amount of detailing you'd like.
Creatures can fair quicker due to the less complicated pieces they may have, humanoids are more complex with individual pieces of equipment that would need to be sculpted out and placed separately.
The good thing about sculpting out figures in digital clay is that we can easily copy and paste pieces for modification, so if you need 4 of the same type of creature in different poses, it would be quick to just modify the sculpt in different stance and do a little detail correction pass.
Unfortunately it would be difficult to give you an estimate on the amount of work you'd like without further details on the type of sculpts you'd require and the amount of original pieces you'd like.
You don't need to put duplicate miniatures into the equation, you would specify numbers while printing.
I'm always looking for work involving miniatures and figures.
You can always contact me through my Email Chriswallace(at)live.com or via PM.
Cheers!