So my uncle is obsessed with yard sales and he is always buying $3 electric guitars. I know nothing at all, but I was wondering how much can you improve these with replacing pick ups and other hardware?

it depends of the wood, the status of the neck, big etc.
but i think that good pickups + electronics + work will be about 100 bucks. so it will be a $103 guitar

Depends a hell of a lot on the particular example of the species, neh?

what did he pick up thus far?

anything cool?