Ebay has gotten pretty hard to find a deal on any more. Check it on a regular basis, but be sure you know what' its worth before you buy. Lots of cheapies and worn out stuff on there too.
I have a 12 inch bit on mine and would never get one smaller. You'd be suprised how hard it is to get the hole exactly where you want it and to keep it straight up and down. Buried rocks, roots and just diverent layers of dirt can affect which way it will go. Once it takes a turn, there's no undoing it.
The other thing to look for is what type of cutting edge it has. This is what your really paying for. I made the mistake of buying a cheap one and now I'm regretting it.
There are several brands out there with replacable cutter teeth that are either hardened steel, or carbide. The advantage is they actually work, unlike my sharpened steel bit that dulls out after a dozen holes in clay.
If you have a good cutting edge, you don't need extra weight or anything fance. Just let the bit do the work and it's easy living. With a bad or dull bit, your struggling for every inch of depth!!!! With sharp bit I can dig my holes in a minute or two, but when it's dull, it takes 30 minutes or longer to drill in the same material. The difference in huge.
Eddie