Have plenty of resistors & LEDs on hand already, leaving off the header connectors for now,
This is how you actually save money building your own Arduino vs buying one pre-built. By the time you have to order every part on the board (frequently from different sites), it's pretty hard to beat the assembled cost by very much. But if you've already got a collection of various resistors and caps and leds, and can decide that other components can be left out for one reason or another, THEN the cost is pretty much PCB+CPU+crystal, which can get pretty cheap...