Slinky I will give you the same advice as I got when I was enquiring about blinds and this came from friends . Don't go for the cheapest as they will soon go out of shape and they won't hang right. I did not go for the most expensive but did not buy the cheapest and I have had them for 7 years now and still hang nice.
Echoing wendilla, I bought some cheap ones years ago and they lasted no time at all. The slats warped, and the mechanism went out of alignment, horrendous.
I didn't go for the cheap 3 for £90 or whatever the price was at the time as they seemed very cheap looking. The last ones I bought were £120 for 2 (£10 knocked off because I bought 2 at the same time). The sizes were approx 8 ft wide by 5 ft. The first ones I bought in 1981 are still in use in a spare bedroom. In those days they cost about £90 for 2 much smaller ones.
Get someone in and they will give you a price .And they will have different prices . If you go to the smaller companies you might get a better deal . Mine came through agents that worked for a company .But the agents fitted them.