I usually judge item worth by its price on the Mann store. Cheaper hat items (i.e. those selling for around $5, I would trade no more than a 1 -2 refined. Your average $7-10 hat I'd trade for 2.66 -3 refined. Craftable hats (i.e. polycount hats) are more common, therefore worth less metal.
In terms of weapons, recently released weapons for which there are no blueprints yet go on average for about a reclaimed or two. New weapons sell for no more than $5 so thats about right. As weapons become more common, they reduce in value to a reclaimed or 1-2 scrap.
You used to have trouble giving crates away, but ever since the strange weapon crates started coming in, people have been trading a weapon for a crate.
Strange melee items are not worth a lot, maybe 2 scrap, a reclaimed if you're desparate. Strange items which can be levelled up quickly, i.e. the mini-gun, ubercharge, rocket launcher, sticky launcher, are worth more, maybe 2 reclaimed even a refined.