GPS Jamming is not very expensive. You do not need to pay US$1500 to buy a jammer when it is simple to build one for about US$5. The problem is that GPS satellites do not transmit much power and are therefore very easily jammed.
Jamming GPS in a local area involves generating a carrier in the region of 1575 MHz. The aim is to use high power to overcome the cost of the design, and to overcome the weak satellites.
Common GPS jammers operate using a crystal frequency standard to ensure that the frequency of jamming is correct. Since Jamming GPS is illegal, why bother about using the exact frequency.
We can then use a much simpler circuit. Something like a MAR-6 or ERA-3 MMIC Amplifier from Minicircuits. Simple get one of these parts, and connect it to power. Connect the output to the input through a piece of CoAx and an optional resistor divider to reduce the feedback power. The length of the coax should be about half an electrical wavelength and can be adjusted to set the exact frequency.
If you like you can also add another amplifier on the output to ensure maximum output power.
Remember that consumer GPS receivers operate on a different frequency from the military ones. This page has only been written to show how much of a rip-off the commercial jammers are, not to encourage their use.