I fully empathize with this - I hate throwing out/recycling electronics. The power side it truly does depend on your local rates of course. If you consider a 24/7 draw of about 35 watts to power an out-of-box 486 DX2 (headless) vs like 3 watts for an RPI, you can do the math with some variance and figure out just how long the $60 will last you. But a 10-fold reduction in power consumption is pretty remarkable. When you consider that an RPI might replace multiple 486's, even more so.
Alternative uses for 486 mobo - incorporate it into furniture design, cut it up to create a wind chime, create stuff like that to avoid the landfill. :)
My parents are still using the first computer I ever built, which is easily 10 years old! Power consumption on that machine is excessive, and I've been meaning to replace it, but the cost per year in excess electricity hasn't made it worth it to upgrade the machine.
How about a reduction in your personal help desk call queue? Or support incidents/month metric? I only partially kid - I switched the folks over to iPads and _everyone_ is happier.
Leaded solder doesn't spontaneously vaporize and become airborne. You have to touch the solder enough to rub some off onto your skin, then put that part of your skin into your mouth long enough to get the absolutely microscopic amount of lead into your body.
Solder went lead-free (and electronics in general went ROHS) so when it's buried in landfills it doesn't seep lead or other hazardous materials into groundwater.
There is zero chance of lead solder getting hot enough to skip the liquid phase of matter and go straight to a gas in a 486.
Alternative uses for a 486 case - make a planter box, bird house or a bee hive. https://www.instructables.com/id/Computer-case-beehive/
Alternative uses for 486 mobo - incorporate it into furniture design, cut it up to create a wind chime, create stuff like that to avoid the landfill. :)