Hmm I think that there's a number of other components that shouldn't be overlooked.

Here's a nutshell of some of what I cover on this topic in my 'What Smart Marketers Know' ebook:

There are pre-sales, fulfillment and post-sales costs associated with every sale made.

In the real world, both pre- and post-sales costs will differ depending on the what exactly you are doing to generate leads. Depending on what you are selling, fulfillment costs can also vary.

Highly targeted prospects that visit your site are likely to generate a higher conversion rate. But these prospects might cost you much more to get to your site than general leads. So many factors can also affect the fulfillment and post-sales costs as well.

So looking at ROI is what you want to do on top of considering conversion rates, but you should also be trying to determine your ROI from different marketing methods.

It's almost 4:30 am here and I'm still UP, otherwise I'd write more extensively. Feel free to ask any q's. This is the stuff I live and breathe

Sharon Tucci
Work Smarter, Not Harder!
The 20 Rules of Online Business Success
http://www.WhatSmartMarketersKnow.com