Generally speaking, normal weapons are valued at 1 scrap, or 1:1 trade for other weapons. There's only a few that cost more; even the extremely popular Machina only goes for 2 scrap. A Manmelter is incredibly unpopular due to the higher utility of other pyro secondaries. If you bought it in the store, people would be sorry for you. Uncraftable weapons purchased in the store are almost inherently valueless, since they can't be turned into scrap by traders.
Your best bet is to go to the tf2 trade subreddit and see if someone has a weapon shop up. They tend to do 1:1 trades for weapons without any added cost to you. If you can't find one, hit me up on steam; I tend to have spares of weapons lying around, and demoknight is most definitely my least played class.