the
PCB
of
I don't know how they do it, but in the Pound Store in Gloucester they are selling a transparent plastic whistle that contains five leds, two red, one green, one yellow, one blue with three button cells powering it, a push button switch and a chip glued directly onto the PCB that merrily flashes them.
It costs £1 (duh, it's the Pound Store), about $1.50.
I guess the lowest cost answer is probably to get a hundred thousand made in China.
Peter