$11 for 60x60cm doesn't sound cheap nope, but then when you consider a metre of bamboo fleece from Snaps Australia (cheapest I've seen it) is $18 a metre (you have to buy 2m at a time though, so $36), it's not tooooo bad I don't think.

And I don't know what the NN flats are made of, perhaps a bamboo material more suited for being used as a flat, as opposed to bamboo fleece?

Can I ask what you're wanting them for? If it's for a newborn, I think you'll find you're changing bub before you soak through a normal flanny flat anyway...
But what I would be doing (and will be doing when I find that bub outwees the flats) is just putting a bamboo booster in the flat. You might find that's a more economical use for bamboo? Just a thought.