Car fan not blowing

For a couple of months now, the fan in our car is not blowing. When I drive fast, air does come out. But not much as it does when I flick the switch to hot. Hot air does come out at fifty mph or more, but no below fifty mph. There is no difference when the switch is at off, one or five, it is always dependent on the speed the car is going. Same goes for the air conditioner, it works when we are driving fast but not slow. I would just like to know what the problem could be, how much it is likely to cost and if it is a repair we could do ourselves? Thanks.