I got the OiOi kriss kross messenger bag and I LOVE it!! It is big enough for everything you need (2 wraps, spit rag, spare clothes, 15 nappies, my 1.5ltr water bottle etc) and I got it on sale for $100.
It's really thick and sturdy and has a separate pram strap so it hangs perfectly on every pram.

OiOi are great quality and value!