I would only pay about $50 per ton over scrap price. There will be many peices that you can't or won't use. Rarely do the nail / screw hols match up to your new perlins. Bottom line, pay no more then you have to. There is something to be said for new metal: multi year warrentee, you can get it cut to length so you buy only what you need, pick what color you want and it is easier to work with.
I'm the one trying to sell it. I am resheeting my polebarn. We were very carefull when we took it off. About 20 years ago I did the same thing and when i put an ad in the paper the phone rang of the hook, it was gone in a day. I guess there is not the demand there was back then.