Not sure if its the most cost efficient, but 4x4in rockwool cubes in a 3x3 OD flood tray with a scalable rez is solid. You can grow as many or as few plants as you would like, and its fairly fool proof.
Not including the lights, or tent, The most expensive part was the tray, somewhere north of $200. I order my cubes in bulk at roughly $1.13 a cube for 144 cubes. I got the big bags of Jacks part A and B, with epsom from Wal-Mart, well over 3 years worth of nutes for less than $150. Pool shock costs $10 for more than I can use before it goes bad in another year or so. Ro filters are personal preference, I got a 5 stage off Amazon for $300, hooked it up to a $30 plastic trash bin with a $5 nozzle at the bottom to fill up 6 gallon jugs to bring up to the grow space (had to set it up in my basement, would not recommend). Got AC infinity gear for airflow at, probably around $350 for a 6in fan, 4in fan, and the 67 controller. Factor in my horizontal tower fan for cannopy air flow at $60. Oh, pumps, hoses, and res bin cost just under $100.
So 1370 roughly for the whole set up (minus lights and tents) but everything i have will last me a very, very long time as long as I keep up with good cleaning practices.
Scrub your res every week with a little bit of white vinegar to keep salts from building up, and do the same with the flood tray between every grow. I soak my food and drain kit in vinegar and then give it a once over with a brush between every grow aswell. You can put the pump and hose in a bucket with a shallow solution of water and vinegar to achieve the same effect, let it dry, and then do the same process with a 10% bleach solution for sterility.
Just my 2 cents.
