I keep have the KTM bag, you need to use some extra velcro or webbing straps so it will hold tight. It's water resistant, add on a dry bag from somewhere like sports direct and you're golden.
KTM must be the cheapest option, but I'll gladly be proven wrong.
Cheapest option for frame and saddle bag right now?
Waterproof would be nice but not necessary.
Any experience with http://eng.uraltour.com/ ?