Summitracing has NGK sensors for under $50. Shipping will set you back another $10, but if you combine other small purchases, I think shipping will stay the same. I couldn't tell from their website if the sensor was universal for either the front or the back. You need to cross-reference their part numbers. Here are the numbers they have listed: NGK-23099 and NGK-23151.
99 Ram 1500 4X4 Quad Cab Long Bed
5.9L, Self-ported TB, 14" x 3" Air Filter, 180 T-Stat, Hypertech PPIII, TPS, AIT, CPK sensor mods,
46RE with Shift Kit, 3.5 Rears
97 Caravan SE, 3.3L, 150K Miles
91 Spirit, 2.5L, 175K Miles and still going strong