As for airwalks with dotty soles - thats what I ride with, currently on their third sole and the uppers are getting pretty shot but they have been amazing, all for £18 (ok the soles are free for me but you get the idea).
Dotty rubber off the bike is excellent on rock, not so hot on wet grass - but thats the same for any of the original soles on a 5.10 shoe or similar.
£30 from FF is a good deal, I'd charge less if you only wanted the halfsole too, the rubber is the main cost with MTB shoes, its crazily expensive even at trade.
I'd be interested to see a shot of the results though and how you feel they ride afterwards. I'd think most walking boots would have soles that are too thick - I've thinned down the EVA midsole and then changed the rubber on a GTX non-bike shoe to turn it into a winter MTB shoe before, that can work well on the right shoe.