Realistically profit made will depend on how efficient you're running your business.
I'd say about 50% to 60% of takings, gross of tax.
Remember you're also benefiting from the fact that you've already got car you use as your own personal transport (unless you got another), and if you want to compare to a normal PAYE job, commuting costs and times are included, though the usual sick pay, pension, holiday benefits aren't included.