Check out Gelert sleeping bags, cheap and good quality.
http://www.gelert.com/products/sleeping/collection
I have one that I bought for the North vs South ride in 2007, same stipulations as you. Pack down small and light for the bike, needed to be reasonably cheap. It has kept me warm on many occassions since and I'd be happy to use it in freezing conditions despite being rated at 'only' 3-season. This one looks good..
http://www.gelert.com/products/pione...0_sleeping_bag