That does not sound too bad. I bought mine from one of the aquamajic group companies and whilst very happy with the rock and getting it cheaper than that 10kg price I could not recommend trying them.
The more you buy the cheaper it does become and it works out just over £8.00 a kilo delivered for the amount you require.
You may get this a little cheaper on ebay from a tank shut-down but that sounds a decent back up. I suppose the larger scale curing facilites the cheaper they can sell it for.
The fuji premium is the best and most porous rock available, this is what I have and can highly recommend it. It does seem very good that they remove the nasties for you.
You will get a little die off ordering over the internet, I did but the cycle finished inside a week so I was very happy with that.