IMHO we shouldn't be talking about ratios here, we need to also take into account rebill rates and the actual price per sale. If instead we look at $/Unique Click, and include rebills in the calculation, that is is a far more useful number.
Here's an example for one of the programs I promote
So total sales is $3366.40
Total unique clicks is 61,657
So $/Click is 3366.40 / 61657
= $0.0545 per click
I find it easier to think what that's worth per 1000 clicks, so *1000
= $54.598 per 1000 clicks