A question about airfares?

When you buy an airline ticket you have the base price and then you have the taxes and fees, but what exactly do the taxes and fees go towards? The reason I am asking is because I am plannng to go to Paris in Sept. next year and the bast price for a round trip is 508, but the taxes and fees are 350...I think thats crazy. I even chose an 'off' day to depart. I booked a flight to London a few years back and with everything it was only 500.