I found an old silver clarinet in my grandma's attic, but it doesn't work because most of the pads have gone bad. Do any of you know about how much it would cost to replace all the pads on a clarinet and maybe a few springs?

Would depend on how many pads need replacing (possibly all of them)

What brand ?

If its in reasonable condition it might be worth getting it done.

If its in a not to good condition you could always have a go yourself.
The music medic repair kits are very useful.
http://www.musicmedic.com/catalog/products/kit-101.html

It was made by H. N. White Co. in Cleveland, Ohio. It's serial number is C 52195. I'm assuming it's in good condition because the only things wrong with it are the pads and maybe a few springs (no dents!). Other than that, there's some discoloration on the metal, but I bet that it could be taken off easily enough.