I bought my extrusions for my CNC from Misumi, they are a little more reasonable on the costs.  See a few pictures here: https://hackaday.io/project/1692-closetfab-cnc-in-limited-space

Definitely interested in specifics; going from wood to plastic to titanium to steel is quite a leap with respect to rigidity requirements! Good luck!

I was thinking T-slot aluminum extrusion bolted together and precision slidey rails, servo motors + belts and sprockets, etc.  I was looking at parts on the adafruit website

From what I've read that type of structure is fine for the first three materials you mention, and will likely suffice for smaller aluminum work too. Titanium and steel are quite a bit tougher to machine though; I'm definitely no machinist, but from what I understand most DIY 8020-style machines don't produce good results with those metals. It's worth asking other folks that have built similar machines, they might give you some good news. Maybe poke around a bit on CNCzone.com?

hmmm, I would think that you could just go slower with lots of oil.

welding together the aluminum might help with rigidity issues... or just switch the whole structure to steel.

sure, I'll take a look at that site