No, it's expensive, but a pretty rational sort of number. If you assume the GTA pays roughly what navies today pay, you end up somewhere around 2-3 billion AUD in todays money (1.5-2 billion USD). That's about a third of a modern aircraft carrier.
Which when put into that perspective, makes the Orion seem pretty inexpensive.
My gripe with that description is that it's a rather terrible way of saying the Orion is supposed to be expensive. There are plenty of other ways of making an Orion seem expensive that the audience can actually relate to without digging into some weird salary/wage chart of service men. Why not give the amount of years it take to build one? Or how many of these vessels are in service? How many Orions can be expected to be built per planet? Heck, I'd be ok with the line simply being dropped.