One of the recent developments is a set of simple, low cost signals that are available from Model Railroad Control Systems. These signals are just LEDs mounted on a silver piece of PC board, but they cost $14.99 for a 12 pack. Leaving out the cost of the signaling hardware, you can have a signal up for a buck and a quarter.
http://www.modelrailroadcontrolsystems.com/simple-low-cost-signals-single-high-head-12-pack/

These look like a good stand-in until I can afford to put in the fancy realistic signals, which will set you back a lot of money.