As another answer stated, we get the building done in 3 days for convenience mostly. The whole thing is amazingly organized and thus moves quickly.
Nothing is done to be cheap. That would be disrespect to Jehovah. The main concern is to not take time away from the builders lives and to continue the preaching work. The preaching work is the #1 priority to us. Look at Acts 6:1-4. A situation arose where the Greek widows weren't getting food, but the Hebrew ones where. Instead of taking up the time of the Apostles with fixing this the twelve assigned reputable men to take care of distribution. What is their motive? Furthering and focusing on the preaching work.
Like the apostles, we too focus mainly on our ministry. We use our resources and time wisely, so as not to slow the preaching works.
I don't know why we decided on 3 days, but we do it and do it well. The level of education doesn't matter. It matters only that you are motivated to do so. If you don't know how to do anything, then chances are you are going to learn a lot. There are a lot of skilled brothers on site working on whatever section is their trade.
We get a lot of volunteers from all over the state and some out of state. Here is a video showing the building of a kingdom hall. This one actually only took 2 days. Quickbuild
Most of us tend to not have a ton of money, but that is because we work to pay bills and provide for what we need. A lot only work part time, so that they have more time to go out and preach.
When building we want 3 things Quality, safety, speed. I think we achieve those remarkably well. Safety. To attest to the quality of the building, just attend a meeting. The building is not extravagant, but also not cheap. The buildings are made to last and are well maintained.
I am currently an unbaptized publisher working towards baptism. I am prospectively looking at going to trade school for something like electrician, HVAC, etc.. sometime early 2014. I will most likely be helping with quick builds very soon :)