You had to ask, so I tried it from scratch. I updated the wiki with build times and cost here. Short answer: 3h 20m for backfire (and 40m for trunk)
My next question is how long the c1.medium instance takes. It's a fraction of the cost and I suspect c3.xlarge is overkill because the 'top' showed CPU utilization at only 10% during the build. I'll gladly run another test but not tonight. I'm convinced there's a better way to do this, which probably involves running 'make' with a script that stops the instance using ec2-api-tools. That way, when the job is finished so is the cost.
I have zero complaints about AWS. They've done a great job in making a simple abstraction of a very complicated technology. I'm actually quite amazed that they've managed to give an average user the power to instantly create unlimited VMs and storage with the push of a button. My instructions don't do it justice, it really is a point-and-click process once you have all of the basics figured out.