I don't think a commissions-based job would be treated any different than a tip-based or seasonal business. The only real way I can think of to determine the actual income is watch it for a year to determine the monthly income. However, in the absence of a year's worth of income, a few months worth could be used. So, for instance if you made $1000/mo for a few months and then calculated your child support, it would be based on $1000/month income.
Some dealerships will pay a higher starting hourly wage since they know salesmen will take a couple months to get their feet wet. But, if you make greater than the full-time equivalent at that hourly wage, you take home only the commissions.