Yeah 10# sounds right. I had a whole bag of southdown in my 15H and it was 5-6". If you ask nicely, lots of people would give you a cup of sand for your tank. No need to buy what friends can give you.
PM sent, I have a bucket of sand (probably not very alive anymore since its been in a bucket for about a month) I can give you some sand from my 120 to seed it.