Main thing is the price versus the quality of paint - a good paint is worth the price as it will last longer on the wall. Are you planning to get a painter to do the work or doing it yourself? Assuming it's the exterior of your house - what surface is on the walls at the moment? Pebble dash / render / rough / smooth, etc. Also, the surface area of the house wall needs to be taken into the costings. Is the house currently painted? If so, are you changing colours?
Keep an eye on B&Q / Woodies for 3 for 2 offers on exterior paints...