depending on car model...
when you put your key to ON position, it closes the battery and IGN circuit. so both have 12V. when you turn your key to START position, you're realying the 12V from battery to the starter motor and bypass the balast resistor to feed 12V directly to the ignition coil.
this setup is for my car... which is 120Y with coil on balast resistor. so i need to find a momentary switch that is DPDT (dual pole dual throw) to feed the 12V to starter motor and ignition coil.
when the button is pressed, the 12V feed to both starter motor and coil. when released the 12V is cut off.. the 12V comes from the IGN wire, so that when the engine is off position, the button will not draw 12V to the starter and coil when pressed.
the best way to know is to remove your ignition key hole and look the connector from behind. use a multimeter to trace the points at every key position.
overall costs? about RM20 for the switch... you'll need to buy a good quality (made in japan) switches as it'll draw quite a lot of current during start up. cheap components will melt. mine currently run slightly warm when startup.
others are wire costs and connector costs. i used a 12AWG wire to wire everything up. got it free from am accessory shop