Actually, it's both simple and complex!
The whole mecanism that throws the hammer to the string, and lifts the damper, weighs quite a lot.
The reason it does is because of the weight of things, the friction involved, and various leverarms.
The key you press is a lever, longer at the part towards you, and shorter at the back. There is a pin that the key pivots around. (sorry if my technical english is wrong!). That is the reason why a little movement of your finger causes a quite violent force in the hammer.
Now, a skillfull technician will consider all things involved, and create as heavy or light action as you want, but there are a lot of things that need to work together.
If you take out a key and look, you will find lead weights attached. They have been put there to balance off the weight. Different amounts of lead for each key.
Arensky said you could attach more leads yourself, at the back of the key. I suppose you could. I also think that a pianotech should be able to help for less than a fortune! You could get a higher or lower degree of quality work, of course. The price quoted above sounds like a total rebuild, including new parts and a lot more than just weighing off the keys!
If you call some techs, maybe you'll get different quotes. I think this mainly will depend on the fact that some of them want to do a proper job of it, while others will do a simple trick or two that they know. Like adding more leads to the back...