I got most of mine from used book stores in University towns/cities. The odd one (usually a lot older) from auction sales.
For some stupid reason, the schools think that the older editions are all wrong, or something like that, so they insist that the pupils get the latest version only, and the old issues are near worthless.
But honestly (personal opinion) the Machinery's Handbook, was rooted in an age before anyone with a shitty cell phone could look up almost any Spec. on the planet inside a couple minutes for free.
In my opinion, look at what the latest Edition is, and start at around $30 for the previous Edition, and start dropping about $5 per, until you hit around a $5 selling price, at which point they are really worth buying, if only to be able to gift to friends.
Which is to say, I really DO think it worth owning for a few dollars. And, I don't really think there is a benefit to owning the most recent copies, unless you really must for reasons outside your control.
It is a collection of References, not a "How To" book. When the internet did not exist, it was nearly indispensable! But these days, it's just another point of reference, to be cross checked against!
I usually suggest that folks that are starting out, get a (out of date) copy of Technology of machine Tools, by Krar et al (which means he didn't do all the heavy lifting!) for similarly discounted rates as I suggest for the MH.