A couple hundred bucks at best. Is the frame rotted where you cant easily see it until you take it apart? Is the engine seized. I do not like buying bikes that were left outside. Rusted hubs, bad wheel bearings, cracked HL bucket, frozen handle bars, bad tires, bad cables, brake shoes that will separate, expensive seat, rotted exhaust, expensive engine rebuild. Both sprockets and chain are garbage.
Just removing the old tires off the rusted rims would take more effort than most can handle or even expect. That means the owner did care what happened to it, so they shouldn't expect much money for it either. This bike would cost way to much to even get rideable. Look for a running/ridable example...
You will be money ahead!