I can't speak to hundreds of miles, but I built a Wifi bridge that went about 1.75 miles across a cow pasture with center-pivot sprinklers using a pair of WRT-5

I can't speak to hundreds of miles, but I built a Wifi bridge that went about 1.75 miles across a cow pasture with center-pivot sprinklers using a pair of WRT-54G routers, DD-WRT, and some yagi/panel antennas from Amazon about 10 years ago. Total price was under $300 and speeds of 11mb were all we went for as it was just feeding a PC for browsing and time-card punches in a shop across the pasture from the office. It ran happily for nearly 6 years before being replaced with a packaged kit of antennas and POE injectors that advertised 54mb over 2-3 miles seamlessly for around the same price. If you had a pile of old 802.11b gear and the time and gumption, I'd think you could build something like this for sub-$50 at this point.