The short answer is that it shoudl be a percentage of your budget. Doesn't it make sense to spend 10% of your budget to measure the other 90% to find out if its working?
The slightly longer answer is of course "it depends." It depends on your goals, because how you measure depends entirely on what your goals are. If your goal is awareness, you need surveys, if your goal is message communication you need content analysis. If your goal is outcomes, you need correlations and statististical analysis. How big a survey, how much content you have to analyze and how many variables you need to analyze depends on the size and nature of your audiences, the complexity of your campaign, and purpose of the research.
The right answer is the one I gave at PRSA Spokane yesterday which went something like this: Today's communications are all about building enhancing and strengthening relationships. The notion that the you should be able to do that for free or for mnimal cost just because the technology is free is ludicrous. And of course 10% of $0.00 is still $0.00. So lets take a big step backwards and look at the vaue of a good relationship.
Organizations with good relationships with their publics are known to have lower legal costs, greater renewals, lower recruitment costs, higher revenue per employee and shorter sales cycles. Customer loyalty has been shown to have a direct link to higher profits. All of which go straight to the bottom line. So if you're worrying about being able to afford to measure that, you're clearly focusing on the wrong things. It's as if you're already in a relationship and you're trying to figure out the ROI of dating.