The sprinkler system cost is determined by the size of the lawn and not the size of the house. Many large houses sit on top of very small lots with very little landscaping. Now a sprinkler system should be designed based on your current water needs for the type of lawn and plants that you need to water.
The price variation that you may be experiencing is usually due to cutting corners or simply putting up sprinkler heads and having rotors water large areas over sidewalks. Many cut corners by watering turf (grass) and landscape beds. This is not usually recommended as different plants have different water requirements. Many of this practices are outright illegal and any responsible homeowner should not consider this type of installation.
1. Size of the Area Needing Water - More Landscaping will require more stations. The more sprinkler stations needed, the more valves required, the larger the controller need for the installation. Of course, square footage will determine the amount of pipe needed and the amount of digging involved.
2. Water Pressure - Low water pressure results in the need for more sprinkler stations.
3. Location and Accessibility of the Water Meter - Not a big factor for most homes. However, occasionally you run into a situation where the water meter is surrounded by lots and lots of concrete floor. There are only two options to reach the water meter. The first option is to dig. The second option is to demolish the concreted area. Again, this is very rare in a residential sprinkler installation.
When investing in a sprinkler system, it is usually best to find a irrigator that follows local laws and best installation practice for the industry. The prices may not be the cheapest but you will usually experience less headaches in the life of the sprinkler system.