For a crude estimate, I'd look at an existing similar property .... and if that other place is more than ten years old, double it!
A few other points to consider discussing with the speculator:
- If the place has less than six units, I would still argue for the extra expense of using a buss gutter. This will save lost of money later, when meters are added, etc.
- Make allowances (extra junction boxes, etc) for shifting lighting circuits and switches around. Lay out your lighting patterns so as to accommodate remodels, as walls get moved around.
- Allow in your bid for exhaustive documentation, and identification, of every circuit- to the level of marking every box and device.
Such steps will pay off big time later, when remodels and maintenance are necessary.
I know, there's likely lots of pressure to do as little as possible, to keep the price as low as possible. You need to resist this. I guarantee that every time there's a change of tenant, you will be moving lights, adding receptacles, etc. When a poor (cheap) job is done at the start, the entire property quickly becomes a dog's breakfast ... and each remodel becomes sloppier, and takes longer.
This is why, for such places, I prefer a 'skeleton' of pipe, with MC (or romex, if you must) 'whips' going into the wall cavities.
