If you do it by your own you can cover it under 4k. Take ordinary bus to bhuntar, after that, hitchhike until Kasol and then to Tosh. You can get a room in kasol in 4-500 per day. Camps at kheerganga are not very costly so it can be very cheaper to you, if you act smartly. Carry food items with yourself so that you won't have to buy them on the trek because with height, the price also increase.