Like Jim stated, this is a very common fault with these Fuel Level Sensors. It is more than likely a failed resistor, or failed wiper in the resistor pack. It can be replaced with out removing the fuel tank. As far as costs go to accomplish this repair I'm not sure. With labor costs (GM Dealership) at $75-100/hr. plus parts, I wouldn't be surprised if the repair cost is near $400.
Good Luck with it. Actually I use my trip meter as the primary means for estimating fuel usage, and the fuel gauge on the inst. panel as secondary indication.
2003 SSEi, Light Bronzemist Metallic - All Stock For Now
IBEW Local 261 Groton, CT
My other vehicle is a nuclear submarine.