A double amphib/transport runs you $6, has an effective range of 9, and a throw weight of 4. To contrast, a Battleship costs $5 has a effective range of 8, and only throws 3.
Quick correction... It would cost 8$. 2$ for the transport and 3$ each for the amphibians. Of course, naval transports do tend to stick around, so maybe they shouldn't figure into it.
I agree, the amphibians are a powerful unit, but the problem is that they are very easy to kill. So unless you can isolate single enemy battleships, etc, they are a very expensive unit that dies in droves. Because they have to get up close and personal to take out the enemy, they are very subject to return fire.
Nothing like taking out a fully loaded aircraft carrier though... and a tranport/amphib combo is one of the only things that has a chance to.
I think that if anything should be reduced, it should be the range of a naval transport. Not only are they the fastest unit in the game, but each move costs only one point, unlike for a land transport. I suppose they are so quick to keep the game moving, but sometimes I wonder if that is very fair. They make it easy to move large forces across big oceans, and even to flank along a coast.