We use a medium weight calico for all our sampling work in college (practising different garment construction techniques), it is indeed great stuff.
I get it for £1.90 per metre I think, or could be £2.50. I too have have made my own catnip sacks with it and as yet, they haven't been destroyed and they've been on the go for months
Just run up on my domestic sewing machine.
I would say the Catac sacks are actually a lighter weight than what I used. Maybe I should start selling them, I do need to up my income and the second job hunt isn't going anywhere at the moment