Go through your paperwork on old houses and get the average sq. ft price.
Ussually when I do it. Its the 'wired to code' price, no extras. Recess, wells, uc lgts ect. all at additional prices.
I havent done it in awhile, last time I did was when copper was at its max price I think I was using around $2.5 sq. ft... lower copper price was 2.2 .. I could be off been awhile