You can measure your online marketing programs against themselves (i.e. compared to last month), or even against industry benchmarks. Both are helpful, and you ought to be watching them as you test/adjust/retest your online marketing.

But you can't allocate your marketing dollars most efficiently with that data alone.

Here's why.

Without more tracking, you can't know if a trade show lead is worth more than a pay-per-click lead. You know it probably is, but how much more valauble? 3x? 10x? It depends on how they convert into sales, and how well you retain them as customers.

You can get more granular. Do some keywords convert better than others? Conversion tools like Google's will begin to tell you, but they only handle the online part of the sale. So if you have people closing sales, the conversion picture is incomplete.
Diagramming the Sales Funnel

The way to measure effectiveness of your lead acquisition and development activities is to develop a complete picture of leads coming in, qualification, and closure. This looks like a funnel, with unqualified leads going in the mouth, and sales coming out the spout.

In the middle of the funnel are several hurdles, separating the stages of visitor - lead - qualified lead - prospect. (Or, any terms that match your sales process.) Acceptable dollar values for each stage can be determined, based on your conversion rates and the lifetime value of a customer.
Reallocating for Efficiency

Once that's done, you can see that (hypothetically) your $1.50 bid for a search engine ad is too high and the program needs tuning, or that it's under the allowable cost and you chould be devoting more money to it. The same goes for banner ads, partner agreements, list rentals, direct mail, and so on. Alternately, you may find that closing a greater share of qualifed leads is a better use of money, since that drops the allowable cost for lead acquisition activities.

This is a vastly simplified summary of a marketing metrics analysis. In reality, there are a lot of rocks to kick over, but the exercise brings discipline to your online marketing programs.