Presumably, the reason vehicle skill books are so ridiculously expensive are to discourage you from going into them before you've got a decent infantry suit under your belt. It makes even more sense when you consider that to effectively use vehicles, you need either way more money or luck than someone who plays infantry. It's unlikely someone will lose a million ISK in a single match playing Infantry, but if the forums are any indication, vehicle users call that "Tuesday." The skill books being expensive, then, are to prevent people who don't have the money to use vehicles from blowing their SP skilling into it.

you only need to buy it once so what are you complaining about, should be complaining how pricey it is to a pilot of any vehicle(beside LLAVs those things make money)

They are higher because they are strong. Infantry can be taken down easy by a good tanker or dropship pilot. It's like for example in EVE, the higher tier the ship is, the more expensive stuff is. You could train for a T1 ship and pay let's say 500,000ISK (not accurate just a random number), then train for a T2 ship and then pay like 2,000,000 (again, not accurate). Because they're stronger, better. Like when you train for the Enforcer, think of it as a T2 tank. it's going to be more expensive because it's supposed to be better. It's pretty reasonable if you think about it. It just kinda sucks because we don't have a lot of options to make ISK. YET!