They seem to be getting rarer - They could be had for about $150, and then $250-400, and now - probably closer to $400. (From a fellow fishkeeper. Retail, if you ever found one, would be twice that probably, here in the US, anyway.)
Growth is pretty slow. I got mine at about 14 inches, 2 years ago - She is now about 18 inches. This is under "natural" conditions - room temperature, an aberrant feeding schedule - so I'm sure her growth is close to what you can expect from a fish in nature.
My longnose, and alligator, have similar growth rate, under the same conditions - which says something, since Alligator gars usually grow very fast when babied / fed a lot in a tank.
They all live together and couldn't be happier.