you cant price the same as domestic, so rule that out as of now if you want a bit of good advise.
when i tend to price for commercial work i always put 10% on top of what its worth, somtimes 15%.
you will get knocked down and you will be 1 of 3 doing the quotation, well thats what happens in my commercial world...
I have 13 and half k worth of contracts a year, not much but its better than nothing and when combined with my domestic its very good money.
My first job i remember saying to the warden, ... call it 250 quid, she said ok no worries just clean the inside of my office window. lol.
still there today