Not expensive at all mate -
LED's from a local electronics store approx. £5
Wires, switch, battery box, resistors approx. £5
Solder, electrical tape, double sided tape approx. £4
Borrowed a Soldering iron from my brother in law, and some kitchen foil (light blocking) from the wife.
No need for those hundred pound odd lighting kits you may have seen on Starship Modeler, or eBay...