Returning for a moment to the transport-pricing question, I think one good way of analyzing that is to compare the cost of sending loaded transports vs. the cost of sending planes.
Suppose for the sake of argument that you have unchallenged control of the seas, so the only extra expense you have to incur for amphibious assaults is the cost of the transports themselves. Also for the sake of simplicity, suppose you have a token beachhead of 3 infantry that is already on the mainland, so that if you support that infantry with enough planes, it can theoretically conquer as many territories as necessary. These two assumptions cut in opposite directions (control of the seas makes transports better; having a beachhead makes planes better), so hopefully they at least roughly balance each other out.
If transports cost 7 IPCs and you are able to use the same transport twice over the course of a tournament game, then the cost of buying and delivering a supporting force of 2 inf, 1 art, 1 tnk is 6 + 4 + 5 + 7 = 22 IPCs. This results in a total force of 5 inf, 1 art, 1 tnk, which has 7 HP, 11 punch, and 15 defense.
Alternatively, if fighters cost 10 IPCs, and bombers cost 12 IPCs, then the cost of buying and delivering a supporting force of 1 inf, 1 bmr is 10 + 12 = 22 IPCs. This results in a total force of 3 inf, 1 ftr, 1 bmr, which has 5 HP, 10 punch, and 11 defense.
Under the (admittedly artificial) assumptions of the experiment, the transports are strictly better – you get more HP, more punch, and more defense for the same amount of money.
On the other hand, suppose each transport can only make one delivery during the length of the tournament game. Buying and delivering a supporting force of 2 inf, 2 tnk will now cost 6 + 10 + 7 + 7 = 30 IPCs. You could instead deliver 3 fighters for those 30 IPCs. The total amphibious forces (including the beachhead) would be 5 inf, 2 tnk = 7 HP, 11 punch, 16 defense. The total airborne forces (including the beachhead) would be 3 inf, 3 ftr = 6 HP, 12 punch, 18 defense. Those forces appear roughly equivalent to me – the airborne force has one fewer hit point, but it has slightly more punch and defense.
Finally, suppose the transports can only make one trip, and they also need to be escorted by a pair of destroyers in order to survive even that one trip. Buying and delivering a supporting force of 4 inf, 1 art, 1 tnk + 3 transports + 2 destroyers now costs 12 + 4 + 5 + 21 + 16 = 56 IPCs. For less money than that, you could afford 4 ftr, 1 bmr. The total amphibious forces (including the beachhead) would be 7 inf, 1 art, 1 tnk = 9 HP, 13 punch, 19 defense. The total airborne forces (including the beachhead) would be 3 inf, 4 ftr, 1 bmr = 8 HP, 19 punch, 23 defense. The airborne force appears superior to me – it would be able to reliably trash the amphibious force if they fought in direct combat.
Part of why I think transports are overpriced in tournament play is that you often do need something like destroyers to protect your transports. You might not be able to finish a second round-trip before the tournament game ends, especially for transports built after turn 3 or so, and you might need two or even three fleets of transports to efficiently ferry infantry from, e.g., New York to Rome/Berlin. If you have to set up a shuck-shuck where transports are constantly swapping places with each other (i.e., if you want to cross an ocean rather than just bridge a single sea zone) then that seriously increases your transportation costs.