As I wrote elsewhere: cheap, warm and light, pick any two. Really light and compact really means a PHD bag. If you want to get something like it for a tight budget, get an Alpkit Pipedream 400. If you want to spend less for a bulkier and heavier bag, get a Skyehigh 600. If you want to spend even less, don't kid yourself. You will get a really bulky and really heavy bag that will not last long. Or you may be very cold.
Sorry, that is how it is.
Willem