RAW, Artisan's Forge does not change or create value.
As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation.
A monster's weapons are generally considered valueless (PHB 144):
Weapons and armor used by monsters are rarely in good enough condition to sell.
Common sense could argue that the metal in the sword should be worth something, but if that was true you would be able to sell them for that amount.
Common sense could also argue that you should be able to use less than 15gp of metal in order to craft a longsword (since the blacksmith's labor should have some value), but that is still not what Artisan's Blessing says, since it goes by the value of the end product, not the value of the materials used.
If your DM wishes to say the metal in them is worth something, then could use the same number of rusted swords to create a new, usable sword as you would need to sell to buy a new, usable sword, so Artisan's Blessing is still not generating any value (though it could be used in a dungeon or somewhere else you would not be able to buy or sell swords).
But going around looting every monster's weapons and armor after a battle is simply something most DMs do not want to deal with.
To answer the additional question that was edited in, by RAW, if you bought 30gp of daggers you could turn them into a 30gp greataxe. However, if you left the daggers out in the rain until they rusted into worthlessness, you could not turn them into a 30gp greataxe, or indeed, even a single 2gp dagger.
Similarly, there's no consideration of mass or size, only a somewhat arbitrary absolute sense of value: if you assume that coins are equal in value to their base metal (that is, 50 coins are worth a pound of that metal, as per PHB 143), you can turn 0.002 pounds of platinum into two pound of copper. Or, you could have a pound of gold, worth 50gp, that is used by a competent metalsmith to craft a piece of jewelry worth 100gp, which could then be converted into two pounds of gold.
That's magic for you.