The proper bearings for a home made spindle that is built for high speed aren't cheap. This coupled with the precision needed to build the spindles and the cost of a big enough motor and controller with the desired qualities are what puts most guys off on attempting to build their own. It can be done. Some guys have and are doing these. Just do a search for spindle designs on here and several should pop up.
The end result though is that you really can't make these much cheaper than you can buy them for unless you have a full blown machine shop already. Then perhaps slightly cheaper.
There is however, always that next great design or building technique that is waiting to be found or exposed. Give it some thought and research. Come up with something that we can make ourselves.