This is what I do:
By "a project" i mean an online for-profit business, a site (one or a few in the same niche). For this discussion, I am considering "internet marketing projects" - sites that run on autopilot or on outsourced maintenance, within a business model of building an empire of sites, each being a stand-alone project.
I'm usually interested to see the setup investment (in my case $100-300, including my time @$25/hr) and time to break-even (that's a month when I don't need to invest anymore, 1-3 months)
The ultimate measure of efficiency = amount of cash you can take out in a month. For the sake of comparability let's take the 6th month after launch. So we are interested to see free cash flow (how much cash we can take out of project) and growth rate.
It is very interesting to break down cash in 2 parts: traffic (new uniques per month) times average cash per visitor. We can, of cause, break it down further, but that's where I usually stop for general "strategic" decision making - to keep the site or sell it off.
Traffic: varies greatly depending on a niche and a business model. I like to see that at least 5k/month of non-paid traffic is achievable. Also, growth is important, I like to see at least 20% per month, cool if it comes from refs (meaning that it's getting viral).
Cash per visitor: most important thing, tho a bit tricky. I measure lifetime value of a customer, discount it by all steps of conversion, to arrive at an average cash that I get from an average new visitor. Of cause i work on my ARPU, lifetime value, cost cutting, list recycling, warming-up incoming traffic, increasing convergences, ect. But it all boils down to cash I get from a single new visitor on average. My acceptable number is $0.1+. This way i can easily estimate efficiency of new marketing campaigns. Example: i am considering a press release, costs $20 to produce, if my cash/visitor=$0.2, I can see that the PR needs to bring back at least 20/0.2=100 visitors to break even.
So my key stats: investment $100-300, up to 3 months to breakeven, non-paid traffic 5k/month+, average cash/unique visitor 10cents+.
What's yours? how do you do it?