The "one dollar" soul coins are usually made out of those who live lawful evil lives and are minted by mammon. The other source is from devils who are defeated on their home plane need and to return to GO without collecting 200 dollars.
The souls that acquired in deals have a greater value (despite what the skeleton says). Deals made by powerful people can start near the top of the hierarchy -- at upper levels say 14 or so you can sell your soul to Asmodus and become a pit fiend on death.
Raphael is both stuck in and free of the infernal hierarchy -- he's half human meaning he can't work his way up the hierarchy but doesn't have to give up souls to a superior to make quota. So if he after souls it's either as soul coins or deals on behalf of someone else. He could be a conduit for another or he just might want to increase his coffers. And it's a bit more than dollar in any sense -- think of whatever the gold coin is for your country.
As to how many? For say a bone devil to move up? I don't think that's ever been specified. I think it would be in the details of the contract with the ruler of the hell.
Edit: didn't see arazhal's post when I replied. Yes, Asmodius would be the most likely end purchaser.
Okay, two more questions. What happens to those who make a deal with a devil? Do they turn to soul coin after the devil fulfill his part, or something else? The wiki says you can free a soul from the coin with "remove curse". What happen to it? Does it turn back to whatever it was before (let's say human) or something else?
And the second question is what about warlocks? Do they turn to a soul coin too?
