Ok first off let me say, I'm an electrician by trade in a union factory. I have been contract labor in housing prior.
it is pretty darn easy to see WHAT is there now, you should have been privided a "bill of materials" for the job durring the start up process. this list should have had $ numbers on it as well as quanity of lights boxes outlets ect... now while you areout in the new barn, take a spread sheet or some other sort of paper, right down exactly what is in ONE building, (assuming the others are similar) count every darn outlet, box hunk of conduit. ect.) take this list and start compairing it to the bill of materials you got from him. Look for obvious mistakes or additions which aren't on the property... make sure that there are100 ights hanging in there if you were charged for 100 if you know what I mean.
Costs for lighting is an area that has really I mean REALLY gotten out of hand... Prices are sky rocketting and what is worse these are imported lights that used to be 20 bucks each are 3~5 times higher now. why? Now sure still made in china all same...
anyhow Copper has also doubbled in last couple years but just recently started dropping again due to a long lasting strike where #1 copper producing mine was/is at.. this is now over and workers are back working, I'm sure with better pay and benifits now.
anyhow also make a list of man hours spent on the job ANY time you have time & materials job. granted some hours are spent making calls getting quotes in the office, these hrs should be negotiable because physical work hrs were not spent ON the job, harder to prove. add up the men there with time spent there in day. If they arrived at 8 am left at 4 pm & ate lunch for an hr then you know each man worked X# hrs. that day.
some of this can not be done after the fact but tracking what was done and when can help you IF and WHEN this gets to the arbitration/legal stages... Don't wait to get this info untill then take it with you to the attorny. take EVERY thing you acn think of to him...
Reason I say this is because one bad contractor can make 10 good ones look like crooks simply by being a contractor...
In the long run you may have better luck trying to work out a deal with the contractor NOW, get the list of what you have installed NOW and take it with you when you go see him. from there go see the attorny of choice... you will have the info with you already... maybe let it slip when you see the contractor at 10 am that you have to be going by 1 pm to MEET with someone and you want to discuss what the bill has and possably work out a payment options with ihm. Be nice and smile

( Oh and try to make sure no witnesses see's you hide any parts

) lol
mark M