Central Vac can be time-consuming, but it's not difficult. You'll get quicker as you do it more, and you become more efficient. The bits are expensive, and you'll quickly learn where best not to drill to avoid nails. (I sharpen my own, and still have the original bits I bought in 1987.) As far as profit goes, you won't make much subbing for someone else unless you get them to pay you what you're worth. Why not just go out after your own work? I have done some subcontracting for different alarmcos, and found that I did the work, while they made the profit. No thanks.
It's a natural add-on, and builders like one-stop shopping. js