That is like saying how much do I pay for a car.
There are a lot of variables and the more you pay generally the better product you get, by that I mean better quality glass that deals with cold/sun better than cheap glass. We have a 6x5m conservatory with a glass roof that is used a lot and is superb. Anglian put it up and cost us late teens but in the three years it has been up not a single thing has gone wrong and still looks superb. Obviously if you are handy with tools you will do it cheaper but if not I think it is false economy to buy a cheap one, if you intend living there for years buy a decent product from a decent company (do research) for a decent price and it will be money well spent.
I would never have a house without a conservatory, we will probably get something like £10k back on the house price so in the big picture not that expensive.