That's just plain wrong socketed mobile CPUs are nearly as expensive as the motherboards with cpu. Even for older thinkpads like the t430 the CPUs are more expensive than just getting a "new" t430. Besides the price motherboards die more often than CPUs that's why you see horrendous prices on eBay for decent old gen motherboards. It's a problem in theory that gets brought up all the time and just isn't true in reality.

I don't want socketed CPUs I want all the innovation and advantage of an M3. I just want to get a chip and either replace it myself or get it done for 50-100$ in a repair shop.

Exactly. The problem isn't the advancement of technology. It's the rampant gatekeeping of manufacturers and both politicians as well as some of the user base defending it.

I can buy a decent desktop i5-3570K on eBay for $10, or two lower-tier laptop i5-3320M for $10 (my laptop came with a 3230M which ruins 100 MHz slower). I'm finding Core 2 Duo mobile CPUs for $12-24 (higher-tier CPUs may cost more, not sure what you're looking for).

And for more than 10 generations old processors that's a reasonable price.

Aren't motherboards substantially more expensive than CPUs? I tried looking for a replacement Haswell Dell Inspiron 15 3542 motherboard (almost as old) for someone with a broken laptop, and saw results in the range of $90.

Dude if you want hardware that as ancient you do you.