Hiring managers screen resumes for certificates; if you don't have the certificate, you won't be considered for the job. In my particular discipline, regulation forbids us from considering anyone who doesn't have the certificate. Companies will cite the number of certified PM's that they're bidding on projects; if you can't help them raise their number above that of the competition, they're less interested in you. (I've heard this from several hiring managers).
I'm not sure that is wise, but that's the reality in which I work. I still oppose the notion of licensing and professional certificates, but the cold hard reality is that unless you can get past the screener and into the first interview, you'll never have the chance to make the case that your experience is more valuable than a certificate.
There is relatively little opportunity cost to obtaining the certificate. Everything you do for the CAPM will transfer over to the PMP.
There are sites that show the difference in salary between a PMP certified PM and an uncertified PM with similar years of experience; I'm unaware of any site that tracks that for CAPM. There are sites that strongly assert the value of the CAPM, and sites that weakly assert the value of a CAPM. Both sites back up their assertions with logical arguments, but I haven't seen any statisitics.