At the moment, Amazon in the UK are charging £16 for the set. That's very good value for money, to say the least.
The US version is currently $27.65, but if you're going to ship it to Blighty there's import fees and postage to pay on top, bringing the total to $34.13, I think. So it works out a fair bit more than going for the British version.
Still, neither is expensive and – review spoiler alert – I did have cause to say, just the other day and about another TV show I can't name right now but I'm sure you can guess, "That's the most fun I've had with a pilot since Firefly."
So. Firefly. It's great, and for Brits at least, it's cheaper than normal right now.