This sounds naff, but Lidl are selling a walking boot for 14.99, i've bought them for a few years now, and they're comfortable and strong.
Each year i get a new pair, and the old ones are still good, plenty of tread and good ankle support.
They are currently in Lidl stores, but are the thursday specials so they won't last long.
Check out they're website.
Here's a pic :