On Quadratec, its $27.99 for a new oil pressure sending unit. I'm sure you could get it at your local parts house for the same price or maybe even cheaper.
I had the same problem in my Jeep. Driving along, oil pressure was fine, but when I stopped at a stop light or whatever, my check gauges light came on and the oil pressure gauge dropped off to zero. I replaced the sending unit after that was suggested on here and it's been fine ever since!