While they will make money from the people that post CVs, much, much more money will come from the companies that pay to search those resumes. I'm sure they set the price at wherever they expect the best filtering to be. If you value your time at $50 and SO careers saves you 4 hours of job searching every 2 years, it's worth it. If it finds you a job that pays you $.05 more an hour, it's worth it. It's underpriced because CV owners aren't the primary customers.
About the decoys, the $99 regular price isn't a decoy. They're offering the $29 plan because they need a critical mass of CVs to make it useful to companies looking for talent. They're sharing $268 of their value to early adopters as recognition that the first people in make the service more valuable. Once they have lots of CVs they're in a stronger position and don't need to discount anymore. Simple business economics.