I can only partially answer this question, and my information is not particularly up-to-date, but I've contacted a friend who has been there recently and added his answers to my own.
In April 2004 I flew Kathmandu-Lukla, and returned via bus from Jiri to Kathmandu. I remember the flight costing about a hundred dollars. My friend flew in September 2013, and he says it cost around $300. He says the price varies from day to day and season to season. Both he (recently) and I (then) booked the flight just a few days - no more than three - in advance after arriving in Kathmandu.
On the way back, it took me five or six days to walk from Namche Bazaar to Jiri. Unlike the area from Lukla to the Everest Area (and the Gokyo area, which I also visited), the trail back to Jiri was relatively empty of tourists, though the Nepali villages were no less bustling. I was walking alone, but I didn't feel much of a risk. Of course feelings of danger are a very subjective thing, but that was my experience.
After arriving in Jiri, I bought a local bus ticket back to Kathmandu in the evening, and took the morning bus, and arrived in Kathmandu that evening.
This was during the civil war / Maoist rebellion. On one hand, that sounds a bit alarming, but the situation did not affect tourists that much (though I'm very glad for Nepal that it's over).
The difference between the area above Lukla and between Lukla/Namche and Jiri was relatively extreme. Above Lukla there were hundreds of tourists around, and it was easy to meet people - I arrived alone and found traveling companions quickly. Below Lukla I met less than ten tourists a day.
The big caveat is that this was a very long time ago.