Rokinon 10mm 2.8 DX Nikon lens
$325 plus $15 shipping
These are about $449 new and $379 used at B&H/Adorama
Rokinon 10mm F2.8 ED AS NCS CS Ultra Wide Angle Lens for Nikon Digital SLR Cameras with AE Chip for Auto Metering (10MAF-N)
Includes front cap and rear cap.
I've had this one for a year. It excels at capturing the grandeur of big places, astrophotography, and flowing water. Here are three shots with a D7000.