Features Versus Size Versus Price
I agree there are some good micro-atx motherboards out there for around 70-100$, however the latest technologies such as DDRII memory 800Mzh, SATA II adn PCI-express rarely comes all in one on a Micro-atx board. There is some boards that has DDRII memory around 600Mhz. (Read asus and gigabyte) and all of the above features. It's also around 70-90$.
My philosophy is to always max out your budget, in order to stay clear of the trap when you computer steeps in value in just a year or so because of the lack of latest-tech features on your motherboard.
If you look at an asus ATX board like some of the ones I'm eying now...
they have features that allows you to build on your computer basis: the motherboard. Plus, the chance of getting a long lasting, error-free computer is better with a good, slightly more expensive (known-brand) motherboard.
At the moment I'm looking at another angle in order to make my next PC more mobile: Finding an ATX case that is very compact, small and goodlooking, instead of buying an m-atx card. They are out there, but I'll have to find out what suppliers buys from these innovative factories.
Cool small ATX cases