Make it worth your while. Your price should plan to payback the cost of your equipment in two years. And it should cover the gas, repairs and maintenance. Pay for labor: by adding your labor per hour, labor burden (tax and benefits), and cost for travel and unproductive hours in the shop. Add more if if skillled (HS graduate), or if you have to sweat or do anything dangerous.
For overhead: say your overhead last year was $1000...and you worked 500 hours last year. You have to charge $2.00 per hour for overhead.
Don't you wish there was an aerator that would plant 10 seeds in each hole?