I always had the expensive brand of sleeping bags. The first two I bought in a bundle off eBay for well under half price. The remainder I bought off eBay or friends etc. Almost all my children's bags were second or third hand. I also sold a few of them after I'd used them or gave them to special people. I always look after things and they were in kept in great condition.
I think buying quality shows. They were always 100% cotton (not polar fleece). And they never had any safety issues with the zips etc. and they lasted, and lasted. I knew my kids were safe and warm.
If you only buy brand new - then research second hand ones, you will realise that you can get half your money back when you are finished.