Diagonal movement is a thorny issue for any grid based game. You basically have three options, all of which are unappealing in some way. Make it cost two and therefore over-cost it, make it cost one, thereby under-costing it or try for some more complex system to better approximate it's real cost. We actually started with the latter option - you pay one for the first diagonal, two for the second, etc. In the end though, we decided the extra complexity and fiddling around wasn't worth it. I think it has been a change for the better.