sounds like a reasonable cost, why so low on labor, do you use a crank type machine, I keep costs in southern california at around 15 for labor, the going rate for professional service and computerized stringing, just wanted to throw in my two cents work, dont sell your services to cheap.
I use a Gamma 7000 ES, which is a electronic stringing machine. When new it cost $1800 and I also string at MatchPoint Tennis shop and have been for the last 5 years!