it doesnt have to be expensive, have you already got the tank?
I have a 10gal planted tank, sand substrate and spent £12 on selection of quick growing stem plants (aim to cover 75% of sand).
With good lighting (consider upgrade to interpet 24W t5 power compact if current set up not very good, and will fit) and DIY Co2 (see pinned thread). Add some sort of plant gro solution once a week and simply trim excessive growth, replant cuttings and withing couple months will fill out nicely, and hopefully with no algae! A a couple amano shrimp to look after things for a while, and soon will have a nice home for a couple dwarf puffers