2000 Dodge Intrepid Why Only 1 Worn Valve?

2000 Dodge Intrepid 2.7L V6. Just crossed 100k miles, replaced sparkplugs (recommended by manufacturer). Problem with engine misfire in cyl # 5 detected after replacing plugs (OEM Champion plugs used). Was idling rough shaking the whole engine/car. Mileage went down to 17 mpg from a good 23-25 mpg town driving. Autoshop tried everything, switched plugs, and coil etc. Took the engine/head apart and found worn out exhaust valve. Other valves/seats were OK per the shop.

Q1. Why only one valve wore out? Any reason? Cheap material? Anything special about cyl # 5?

Q2. Also, it is costing me about $ 1300 to have it repaired! Am I being ripped-off or is this what is would cost for having the valve job done? An outside machine shop is also involved as the auto shop does not do the machining, probably nothing unusual.

Car has been regularly maintained at the dealership's shop since it has been bought there new. Small town with < 15 miles daily commute total. Has been a reliable car with no problems so far until this 100k crossing.

Thanks for your time.

PS: This case gives credence to the 'rumor' that American cars are designed for 100k miles?