OK - my cousin has a 10 year old Sears lawn tractor in her garage - the
sucker looks in mint condition. She paid $1500 for it when new, so is it
only worth 25% of what she paid for it?

Basically yep , about $425 or so. If she's lucky she might get about $800
for it if she was to sell it. The resale value largely depends on what new
retail prices in your area are, just like with cars, trucks and motorcycles

Snow...