Actually, the majority of their costs come from direct agent fees and commissions. These fee costs are lower for transactions that originate online, but most don't; of transactions that do originate online, most are terminated by agents that pay out in cash, and are compensated for that. Compliance costs are huge, but they are a fraction of agent fees. It might be said, however, that a good percentage of those fees are to compensate the agent for their compliance activities; a lot of the Western Union's customer contact is handled by the agents, including compliance events.

All of that being said, Bitcoin eliminates or at least reduces many of those costs, in that delivery and receipt of payment can proceed without the involvement of the agent. And FinCEN, with its determination that institutional bitcoin services must be handled by a money transmitter, has just created a barrer to entry that WU is well inside of.

I concur with you, however, that interbank fees are a minimal component of their cost. Also, transfers processed by WU and its competitors are usually instantaneous from the point of view of their customers (part of the value they bring is the massive network of paying and receiving terminals that exist worldwide).

Source: I was the head of IT and operations for a money transmitter in a previous life.