I've been using Aquasoil for several years now and I simply love the stuff (especially the powder type). I've come up with a rough formula to determine how much is necessary based on area: four liters per square foot of tank footprint. I prefer a somewhat deep bed so you can probably get away with three liters per square foot, but it might be a bit shallow and make it difficult to keep some of the more buoyant plants rooted.
I'm guessing your 30G is 30" long by 12" deep by 18" tall. If I'm right then you've got 2.5 square feet of footprint which would need from 7.5 to 10 liters of Aquasoil for proper coverage. The large bags of Aquasoil are nine liters and the small bags are three liters. So if you've got half of a nine liter bag then you might be able to get away with purchasing one small bag (just $10 at AquaForest
). If three liters is not enough the nine liter bags are $26, not a great deal for the best plant substrate out there.
AquaForest also carries the new Aquasoil Amazonia II which is designed to reduce the cloudiness often associated with using regular Aquasoil, especially in places where the tap water is alkaline. The price is the same and ADA claims that it performs just as well as regular Aquasoil so there's no reason not to check it out if you want to avoid cloudy water.
Hope this helps