I wasn't going to comment on this thread because it's been so long since we built that I'm totally out of date on the /sf costs.  But then you mentioned the magic word:  bid    
To the OP:  Please be very vigilant if you put anything out for bid.  It's not automatically a bad thing, but can trip you up if you aren't on top of your game.  Make sure your specs are very clear, very specific, and well spelled out.  As you can probably tell, we got caught on this one, 36 years ago.  The plumber's name is still a curse word in this house.   We didn't have much money when we built, and bids on the major systems seemed like the way to go.  We weren't specific enough.  The plumber with the lowest bid did everything on the cheap.....cut corners, cheap faucets, cheap materials all around, idiotic lay-outs of lines, etc.  Ran the drainage tile from under the foundation into the septic tank because that was the shortest distance that took the least materials.  Tree roots really love the tiny little settling gaps in those lines.  This spring, we get to tear up a big portion of our front yard to fix that major cluster.  Anyway, just be aware if you put system work out for bids.  :~(