Artisan's Blessing can make coins (but not at a profit)
From the rules, I count four criteria an object must meet to be created by Artisan's Blessing. It must be:
- A simple item.
- Made at least partly of metal.
- Limited in value by the value of the input (max value = 100 gp).
Common coins are simple, non-magical items that are made of metal and are worth less than 100 gp, therefore they meet the requirements.
A pile of coins is also legitimate (up to 100 gp worth). The example listed in the rules of ten pieces of ammunition is a precedent for creating multiples of a lesser value item. Even a mixed collection of coins is reasonable, as this is no more complex than other listed examples such as a set of tools.
So, how much do you get?
There is some grey area regarding how to assess the value of the metal being used as input material. The main conflict being between whether its value lies solely in the amount and rarity of the metal (as if melted down to ingots) or if there exists some added value based on its current form (be it crude weaponry or finely wrought jewelry).
The inclusion of coin as a viable material cost provides some guidance. If you could only lay out coin (like a tithe offering) then the value of the input and the output would always be exactly the same and the two would also be of the same worth to the cleric. Following this line, the value of input other than coin should also be what it is worth to the cleric. If an item is worth more to the character in its current form than melted down, then it should be worth more as input material. Similarly, if it's worth nothing to the cleric then it's probably worth nothing to the deity (DM), who ultimately has the last word on the matter (and tells you what it's worth in the first place).
Personal approach: As a simplified approach I would give them half of what a merchant would on most metal containing items. This would cover some of the grey areas like how much non-metal there was (the bulk of the value in most of these things is usually in the worked metal anyway) and how much is it worth to the cleric to not haul it to market or find a buyer. Art objects retain their value at market and none worth 100 gp or less would be trouble to carry, so these would probably not be worth transmuting.
If a cleric wanted to haggle with the deity, have at it. But they should be warned, not every God of the Forge likes having their judgement about such things questioned. For how much trouble the cleric saves not having to lug the stuff back to town they should take anything they get and like it.
All that being said, a DM could simply rule this out. Perhaps the kingdom's currency is magical or the deity just doesn't like being used as an ATM. Gold bars (measured by the pound, 50 gp per pound) are a trade good of the wealthy and retain their value as you surmised. But, as trade goods they might also be considered currency by the DM and similarly restricted. I suggest creating small gold or silver rings and bracelets worth whatever you have to transmute. These retain their value too, and also easy to carry.