Artisan's Blessing handles rusted and mistreated weapons the same way it handles everything else - based on the worth of the raw materials they contain.

De acordo com a Artisan's blessing:

you must lay out metal, …, with a value equal to the creation (XGTE)

The spell isn't interested in the value of the items you lay out, it's interested in the value of the metal that they are manufactured from. This is not the same thing. For example (credit to Dale):

'A new sword is worth 15gp (7.5gp used) and weighs 3 lbs - most of which is steel. That is the value of a longsword [and a large part of the cost will be based on the workmanship that's normally necessary to craft it]. How much is 3 lbs of steel worth? Say 1gp maximum.'

So, rusted weapons certainly won't be worth the item's full price, but they shouldn't even be worth the half price that you might normally get from an in-game vendor for a usable second hand weapon. A heavily rusted weapon is no use to anyone except someone that wants to make use of it's raw materials (ie. a Forge Cleric).

Exactly how much these raw materials are worth is going to be up to your DM.

Não.

CRU, 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 Bênção do Artesão 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 Bênção do Artesão 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.

I don't see why it shouldn't work. It just says

As part of this ritual, you must lay out metal, which can include coins, with a value equal to the creation

so if you have 30gp worth of metal in any form, you can turn it into any form worth 30gp.

Now in terms of whether you can remove rust and damage from a weapon, if I were your GM, I would allow you to remove the rust for free, and depending on the level of damage, I would have you use about 5gp worth of metal to repair a moderate amount of damage.

you must lay out metal, …, with a value equal to the creation

Assim:

1. Are your weapons pure metal? If not, you must first strip the non-metal off them. (I'd argue rust counts as metal)

2. What is their value?

In D&D, the game assumes things have an absolute value.

The value of a rusted crappy sword is that of a rusted crappy sword, not that of a clean sword. If you clean the rust off, the value of the remaining sword remains a constant.

It doesn't say 'scrap value' or 'value as raw metal', it asks for the value of the things layed out.

You can have 10 gp in raw gold carefully crafted into a 100 gp value figurine. You can then use the Artisan's blessing to turn this into a 100 gp bar of gold or suit of armor.

Similarly, if those rusted swords are worth 1/4 as much as a normal sword after you strip the non-metal parts (like the binding or the grip), you can use Artisan's blessing to convert it into a gold bar whose value (in weight) is equal to the sum of the weapon's value. It'll be reduced by the damage the weapon has taken, but it will be far less bulky.

Based on the description of the spell

The metal irretrievably coalesces and transforms into the creation at the ritual's end

From my interpretation of this spell, you place some metal into the space of which you cast the ritual. That metal is then reformed to create the new item, so it would inherit the state of the material it was made from.

You can create something worth no more than 100GP. This would be worth 100GP in perfect condition. So when putting the limitation on the spell it must be worth that new, but that does not mean your item will be worth that

Because you are using rusted and tattered object to create this axe, the axe you created would not be worth 30GP as it was created with rusted metal and would be rusted itself.