I wear a fitted mock under armour that i used to play softball in. Its from their "all season" gear collection where it is supposed to keep you warm and dry in cold weather but also works in hot weather too. ive never had any problems on the mountains with it. But i live in socal and ive never ridden in anything less that 40 degrees lol
anyway, under armour is awesome. but if i didnt already have one, id probably be fine with a $10 thermal from big 5.or sportschalet. you really just need the sweat wick-ing action. anything is better than cotton
I haven't tried UnderArmor stuff because it is 4 times the price. Both the UA 1.0 and the Starter stuff are 100% polyester so I can't imagin that big of difference. I can also say that I wear them all day while working. You can also find stuff at Sams that is Poly and wool and about twice as thick for $20. I have one of those shirts for colder days.
The Walmart stuff will work, but the Under Armour stuff is a cut above in terms of fit & finish.
If you are looking for something that will just get the job done for cheap, then there are plenty of polyester shirts that can be used as a base layer. However, if you plan to keep your gear for a few years and maybe use it for other non-snowboard purposes (layering up when it is cold out, etc), then I would suggest trying on the UA/Patagonia/etc stuff.
I find it to be more comfortable and it seems like it will hold up better than the bargain brands.
Don't get me wrong, UA is overpriced... but if you can find it on discount (marshall's, TJ maxx), then its a fair price for what you get.
Omniwool is the stuff from Sams that i have. 2 layers, 20% merino wool/80% polyester outer layer and 100% polyester inner layer. $15 a piece at Sams this year. It's also what my friend wears when he is delivering 1000 cases of beer at night during winter.OMNIWOOL Multi-Sport Hiking
if you have a target or marshals nearby you can look there. They have the special base layers made for riding for 9 or 10 dollars. If your no riding in too cold weather like out west get the midlayer one. It'll keep you warm and no sweaty. It the compression shirt and pants though. If you're riding out east where it freezing as hell get the a heavier baselayer or you can wear the midlayer stuff and just wear some sweat pants over them. Thats what i do but my pants are more of a shell and get pretty cold.