What are the factors in determining the cost of a septic system and what can I do to decrease it?
Costs range in price due to: 

1) number of bedrooms: You're allowed to have 100 bathrooms plus the worlds largest living room, but if you have 3 bedrooms instead of 2, you'll pay more because the system is based on the number of occupants...and that's determined by the number of bedrooms. 

2) Percolation test: "Good Soil" will decrease your cost because it will reduce size, depth, etc.  To insure the "best" test possible, I'd test in several locations...but remember you need a backup site too. 

3) Competitive Bids: The best thing you can do to reduce your costs is get at least 3 bids...check references...make sure they're licensed...and don't pay upfront, pay when it's done AND APPROVED by the local agency.