I buy kilo bags at around £3 each. I use one bag per tank and I have two snakes that are on aspen; but I do make the bedding pretty thick. So it works out around £6 per clean-out. If a 11 litres is more or equivalent to 2 kilos, therefore, that sounds pretty reasonable, but I can't convert it for you; my maths is terrible. :grin01:
If you want a REALLY cheap substrate, you can use paper towels or newspaper. But if you want aesthetics too then aspen's probably your best bet.