I'm afraid cheap is the way I go. I am currently on a mammoth sack of B&Q regular potting compost, but I must confess, it is pretty chunky so I am sieving it as I go for seeds. If I want to beef it up for potting up bigger plants, then I do about 50/50 potting compost and my own compost or leaf mould, with maybe a handful of grit or sharp sand....if I have any that is. Check out your local nurseries, ours often have specials on....3 for the price of 2 for example.