My job description grows everyday. I have just recently been asked to research marketing techniques and incorporate them to boost our internet sales. It was agreed that I would work on this during my spare time at work and also outside of work. I was asked to write a proposal that reflects how I would be compensated for this. I am at a loss as to how I would figure this. I could figure an hourly amount and create a salary out of that, however if my efforts do generate growth in sales I would like to have that considered in the compensation. I would appreciate any ideas.

That's the most silly thing I've ever heard of. Make the proposal give you a big salary increase. After all you're not being compensated for the work you're doing outside of work.

Sounds like your HR department and boss are too lazy to justify a raise so be your own advocate and prove your point.