Cost of Stair lifts?

I have been looking at bungalows and flats but the value of a house seems to so much better. Does anyone know what stair lifts cost and the cost of having them put in. I know it's a piece if string thing but a rough idea would help.

Also I have a feeling that my stairs may be too narrow where I am at the moment. Again, can I pick your brains if anyone knows what is "too" narrow.