Here's the CDC's most recent recommendations (from 2019) https://www.cdc.gov/mmwr/volumes/68/wr/mm6832a3.htm

The justification for 27-45 year olds heavily references a meeting. Based on time, author and title, I think either https://stacks.cdc.gov/view/cdc/78082/cdc_78082_DS1.pdf or https://pmc.ncbi.nlm.nih.gov/articles/PMC10395540/ should be a fair summary of the meeting (I hope...).

I don't really have time to read it all, but the basic idea is as you said - the cost-benefit ratio is off. Basically expanding from something like the current case, to vaccinating up to 45 year old will avert an extra 21k cases of cancer (compared to the base case of 1.4 million) - so about an extra 1.5% cases averted, while the direct vaccination costs are estimated to increase from 44 billion to 57 billion (+29%).

The current guidance says "do not recommend" plus "consult your doctor". You should read that as "blanket vaccination as public policy is cost inefficient in that age range" not "you as a 45 year old should not get the vaccine categorically".