Little Mum,
I think it's a pretty tall order to get a low price on organic baby food--especially in Hong Kong--my experience is that nearly everything I buy in HK is about 3 times more expensive than I would pay in my home country for the same product. I think this has something to do with the fact that most of those types of products are imported from somewhere as HK doesn't produce much of it's own food. And organic stuff is kind of a "luxury good" as far as food goes--in HK also most stores usually only carry the upper-end brands for that type of thing--whereas in my home country they also have store brands that are certified organic. I have seen some organic baby food at a Park n' Shop in Chai Wan, though--so I think depending on where you look you can find it and the markup isn't as high. I doubt you'll find a truly "low price" on organic baby food in HK.
Having said all that--check out the local organic food growers in HK (there are quite a few in the New Territories) as well as local Farmer's Markets. You can actually learn how to make your own organic food--for your baby and yourself and then only use things like boxed cereals that you can't make yourself. Hope that helps!