Thrift stores are a good place to find base layers as is wally world and the like.
I use under armour which works well and so does anything with a polyester/spandex/wool combo.
I have rode in temps at -30/-40 below with the windchill factor and have been fine.
The biggest thing is not to over dress (you should be a bit chilly when you start out or you will quickly overheat and be in trouble if you have to stop for repairs).

The most important thing is to keep your extremities warm, I use wool socks, mittens and a polyester, nylon,lycra blend balaclava.

I refuse to pay bike shop prices just for the "name brand" so I agree with you.