Personally I have never bid a job by square foot. Since your in unfamiliar territory I would figure out this job with a take off of materials. Then figure your pipe bending skills and labor.
I bid in units. Which means I bid a receptacle to include a box, ring, recept, 20' of cable and all the little parts + the labor to install it. You may not have the time to figure that all out before bid day so here is a thought.
Home Depot gives away a CD that has a little estimating package included with a catalog. I have found it to be fairly accurate on average labor units for any given item. You may spend a night figuring your job out using it but hey it's free and you can use it in the future.
With regard to what the others have said about what is required in the barn and how it is wired. In situations like this I often cover myself by having a line item on my bid that says "Bid per drawings and notes dated x/x/xx sheets E-1. E-2" etc. If things change you have an out.