I think it would be pretty tough for me to sell my marginal hours for more than my hourly rate.
I'm sure if I shopped around aggressively I could find a richer buyer for the 40-hour bundles I'm currently selling, but people aren't really beating down the door for a couple hours of C++ dev time here and there. Especially once you factor in time spent on lead generation, negotiation, dealing with collecting payment, etc, I think the market value of my free time is probably way less than my hourly rate.