The higher loft down bags will normally compress down to half the size of a synthetic bag and can weigh half the weight but normally cost 2-4 times the price.

The trick is to get the down bag on sale. You might even find them at unusual retail outlets.

I have picked up a couple of excellent Vango Down Bags (2 and 3 season bags) at half their RRP i.e the price range for a good synthetic of a similar temperature specification.

Keep an eye out for the down bag bargains.

The local Tesco supermarket chain over here in the UK, carries a cheap Down Bag for around $60 but with some caveats. Its a narrow bag suitable really up to a 40 inch chest and would be comfort rated to around 5 Degrees Celsius or 40F so really a 2 season bag. It is small and compact though and almost competes with something like a Vango Venom 300, which normally costs 3 times as much with its 650 fill power (UK) or 700 fill power (US). Weight is similar to the Vango but the Vango will get you another season for the extra $15 it cost me.

Edit - I've also heard that the Tesco down bag has occasionally been on sale for less than $30. But unfortunately will be no good for me as the bag is just to small.