Great idea, but I still think wireless might be a better bet, it surely involves a lot less digging.
I suspect it depends on many factors. If the Arduinos are battery powered, I could imagine the drain of doing wifi might drastically reduce the amount of time the batteries will last. You probably don't want wifi, which 802.11g is something like 95m best case for access points (according to wikipedia). I don't know if there are Arduino shields that do 803.11n instead of 803.11g which seems to give you more range.
You can get some Ubiquity Nanostations (directional Wifi Radios) that will do 10km easy, assuming the two points are high enough off the ground.... Cost is about $50 per radio, you can probably find some used for less. You could also try 1 nanostation with an omnidirectional antenna, or a picostation. The ubiquiti gear is also power over ethernet, so you can power it remotely (and probably steal a little to power the arduinos too) or you can skip the POE injectors and run them off the solar panel's DC voltage.
Depends on the situation. Direct burial cable is pretty expensive, add in serial modules on each end, the time + cost of digging. If you live in a climate where you don't have the bury the cable very deep, you could probably get away with cheaper cable, or cheap cable + cheap conduit.