Take your best estimate, double it, add 20%
that puts you in the ballpark. :-)
See if you can find a construction ...... (arrrghh, can't think of the word), anyway, instead of bidding teh whole job, they charge you by the hour anytime they are around, and add 10 to 20% to the sub's bill for their time and setup and gaurantee.
This way nobody shorts trying to make a profit (since they fixed bid it) and you can pick and choose the subs he presents and what level of things you want. Generally, they may let you get some of the materials yourself (although no gaurantee obviously)
I've done this twice (same guy) and it's been very effective. He knows he's making money, I know I'm not getting ripped and I don't have to be aroudn to watch the subs do work. (and he does the carpentry).
But seriously, guess high and add some coverage, you will be off.
and don't pay til final inspection is passed.
Generally, most contractors want any money for special orders (which seems fair) upon signing. Then half upon passing some rough inspection, then the rest upon passing final inspection. Sub's won't give a crap about you (regular one guy) but they do care about pleasing the general/construction...... who will give them more business.