Ruger doesn't make the MK III and Mk II in 22/45 NEW. The 22/45 is it's own model now. (That's why I recommended the 22/45.) Add, IIRC, Ruger hasn't made the MK I or II NEW for some time.
I do believe one can get the MK III STANDARD NEW for the price mentioned or less though. But, probably not the other MK III models. I'm only talking new here.
My 22/45 is the 5.5" Bull Barrel and I paid $249 for it in November 2008.
P512MKIII 10107 Blued Alloy Steel 5.50" 9.50" 2.06 lbs. 1.00" 5.35" $326.00
P4MKIII 10109 Blued Alloy Steel 4.00" 8.00" 1.94 lbs. 1.00" 5.20" $326.00
KP512MKIII 10110 Stainless Stainless Steel 5.50" 9.50" 2.19 lbs. 1.00" 5.35" $435.00