Price per mile

I'm with others - that's a big risk for a small reward to do this on the side like that. My guess is that any accident would be declined by insurance unless you were registered and insured to do it (which comes with extensive costs of it's own) - and I wouldn't count on Joe Craigslist to lie to an insurance adjuster as to why you were towing his/her rig. That $100 weekend gig could end up costing thousands if things go awry.

IMHO - it's an all-in or all-out scenario. The middle ground is just too risky for me.