As above. Dont worry about the running costs where pistols are concearned.
Most people i know shoot .44/.45 as apposed to anything else, theres more choice of guns in the larger calibers and if you have to rely on buying your balls they seem to be more available. Best to try as many guns as you can, make your decision as to what you want then look at whats available in that model. Hold a .44/.45 ball in one hand and a .31/.36 in the other then decide Anyway at my age i need the bigger hole in the target to see where the balls gone!
Have fun. Tim