Iride01 

You should get as many as you can comfortably afford. Unless your desire is track riding in a velodrome. In that case you just need a fixie.

The important part is not so much the number of gears, but the range from low ratio to the highest ratio. You need to make sure that the low gears will get you up the hills you might normally encounter where you ride. And that the high ratio will let you go as fast as you want... typically down the other side of the hill.

For normal speeds on level road, I like to be in my big ring on the front and somewhere 3/4's of the way down on the higher ratio cogs (the smaller cogs) on the rear.

As for why more gears cost more, well new tech cost more. 5 speed rears are older than old. Their design and technology was proved out well before the turn of the century. 8 and 9 speed rears are old tech too Trying to cram 12 gears on the rear is the new bleeding edge tech that demands more money. It takes some designing, engineering, new materials and manufacturing processes to fit all those gears on the rear and not have to change up everything else about the rear of the bike.