When you pick up an object on the ragdoll screen in the battlescape, it will show the TU cost to move it to each possible location (eg. your backpack, your belt, your hands, the ground, etc.)
However, this is misleading when it comes to reloading ammo. If I have a gun in my hand, it will list the TU it would cost to move that ammo to my hand if my hand were empty, which isn't actually what I pay if I click there (and reload.)
Therefore, when you have a weapon in your hand and are moving ammo, I think the TU cost for that weapon's slot should show the total TU it will cost to reload that weapon with ammo from whatever location you're taking it from -- that is, the total TU cost you'll pay if you click there.