When I purchased my Turkcell dongle there was a special offer of 99tl that was for the Dongle, Sim and 4gb for one month. After the month you just pay to top up - you can top up with 1gb or 4gb for a month or you can get 4gb for 3 months or 10 gb for 3 months.
However as you will see from Graham Taylor's post the price is now 139tl. I don't know the current prices for topping up are but I'm sure somebody will be along with the information shortly.