I built a lower a few months ago and while it may not be NECESSARY to have specialized tools, they CERTAINLY help! I'm talking about a vise block to hold the lower, the special punch for the captured pins, a hammer drop block for testing the trigger, the appropriate set of punches, and the buffer tube wrench. I get the FFL discount from Brownells (my C&R license has paid for itself MANY times over!!) which helps, but all those tools still adds up. I don't ever plan on buying a complete lower again, so it was a good investment for me, but wouldn't be for a single build.
One thing that really ups the cost of a home build is adding in the go-fast parts. If you can limit yourself to the cheap kit you will do ok, but once you start tossing in match triggers, TD grips, ambi switches, mil-spec parts, etc, it really adds up, though it is still cheaper than adding those things to a stock AR down the road.
dmftoy1, I don't think optics are allowed in DCM, at least not in the regular matches. I'm not sure a flattop with a carry handle would be allowed either, at least I've never seen a DCM rifle offered with anything other than the A2 receiver.