PPC budget strategy?

Best practices for an e-tailer

[Point_It]

This question is for larger sized B2C ecommerce PPC programs.

What percentage of margin do you allocate for the PPC budget?

Given that strategy dictates tactics (driving market share vs. maximizing ROI) and consideration of life time value, I'm curious what are others doing? If you don't mind providing the vertical market you're in as well, it would be much appreciated.

Thanks!