I honestly don't know. I've seen similar pants go anywhere from 2500 all the way up to 10000. Whatever you think is reasonable - if I don't snag them, somebody else probably would :)
You go to "edit auction" then "advanced options" then "add GIN" then enter the amount, then click update or save changes. It's totally ok if you don't want to - I'll be watching and bidding either way :)