You should check with your professional association, who typically offer "suggested" rates of renumeration. A cost analysis on what the market would bear would be a better approach, no?
In most cases, consulting engineering requires by law that liability insurance, workman compensation (for offsite entry as part of your work), company registration for government (federal)/provincial or state (local) legislation, permit to practice (professional designation and defined scope of work as your expertise) and of course, financial institution statements that cover net worth, identify company directors as officers of that business practice (corporate or limited partnership), the list just keeps on going. Having the government tax department nailing you for income tax evasion wouldn't necessarily constitute "party time".
Afterall, machine design is something that affects public property and health/safety/welfare.
You just don't start designing machines because it's so much fun to do so....
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada