If you need a power mitre anyway then buy one. I did also. However the first time you go to make a frame you will realise why you need a proper moulding cutter.
I've chopped quite a few frames made from frame stock purchased from a discount picture frame warehouse here in Atlanta. It ain't rocket science. Having said that one won't have the nifty corner notches to insert joint biscuits or splines that chop shops can make. However there are solutions for that too.
If one has the time precise frames can be made. On the other hand I've sourced really cheap plastic frames from the internet when I've needed to save money and time.
One of the best workshops I've ever taken was a framing and matting workshop given at a local community center. For $30 it was money well spent.
Lastly search youtube for very informative videos. The techniques shown will save you time and money.
But having said all of that try finding a chop shop that can serve your needs, you might save time, money, frustration and maybe your fingers!