I would also suggest, as some have hinted at, that labor required be taken into account. Mining iron is hard work, for sure, but making steel is a lot of work also, you have to combine iron and carbon (usually from hay or wood) and it takes hours and hours. Then to make a sword requires many many more hours. A sword has a certain weight of around 3-5 pounds. So 5 pounds of sword is worth the same as 5 pounds of clay (used for pottery). Not very likely because of all the labor involved. You might be able to say that 5 pounds of sword is worth the same as 5 pounds of shovel heads. So what I'm saying is items further down the production chain (finished goods or manufactured goods) would have to be worth more than items at the beginning of the production chain (raw materials) due to the labor factor. What happens if one blacksmith makes better swords than some other blacksmith whose swords always break? I doubt that they'd be worth the same. What if some baker makes delicious pastries while another baker just makes flatbread or one tailor makes fine clothes while another makes burlap smocks?

Since your land has castles that implies a nobility and those snobs like rare things like big fat gems and gobs of gold. Sure they may be high-minded enough to really care about their subjects but they're not going to level the playing field and live like the rice farmer when they've got a big castle. Gold and iron also weigh about the same but one is going to be prized by the upper crust more than the other. If I go to the market to buy a bagload of potatoes I'm not going to give up a couple of swords or bag of silk shirts. The farmer might say that his work is just as hard as the blacksmith's but I eat the potatoes and they come out the next day but the swords will stick around for a few years. I can also defend myself with a couple swords - I don't think that I could defend myself with a bag of potatoes unless I was Bruce Lee. So object permanence also figures in to the equation. If I'm the guy living in the castle I also want some salt and butter on my potatoes, and maybe some chives or dill, so I'd rather cough up iron dust instead of steel dust. The Dals in the Mallorean presented a utopian socialism to the world but they still had a heirarchy where the seers lived up in the mountain and told the others what to do.

You might be able to get away with this on a small scale like a village here or there but economics will always win out.