@barney:
The official range of a naval transport is 2 sea zones, but the infantry that load and unload from that transport can move through 4 zones. E.g. infantry starts in London, loads onto English Channel transport for free, then the transport moves one zone to French coast, then the transport moves a second zone to west coast of Gibraltar, then the infantry unloads for free into Morocco. Morocco is 4 moves away from London, but the 2-move transport can still make a delivery there. Even better, the 2-move transport can then remain in place just off the coast of Morocco, without needing to look for a landing site.
The air transport can move up to 6 zones, but it takes 4 of those zones just to make it to Morocco, so unless you have somewhere safe for the transport to land that’s only 2 zones away (e.g. Gibraltar), then the air transport can’t land anywhere after a combat move to Morocco, and so the air transport can’t deliver any units to Morocco from London on a combat move.
If you’re talking about strictly non-combat moves, then, sure, the air transport starts looking somewhat more cost-effective…but if you already have a beachhead, then you usually have other options for delivering reinforcements, like a minor factory, flying in fighters, building an airbase so that you can scramble fighters to protect incoming sea transports, conquering an adjacent territory to block the blitz, constructing a bunker, etc., etc. I might occasionally send a cargo plane to reinforce a key territory of my plane wasn’t busy, but the reason I would buy cargo planes is to invade and conquer valuable but unfortified islands like Malta, Crete, Java, Solomons, etc.