1) Thrift stores. Be patient, go through all the sweaters and read the tags. There are a fair amount of Italian brand sweaters with very fine merino weaves that do an adequate job. They are not cool camo colors and are not as thin as the fabrics used in most of the hunting offerings. However, at $3.99-6.99 for a top that'll do the job, I don't care what color it is. My only base layer for two weeks of elk hunting this year is a black merino sweater tagged $3.99 and it was a 50% day, so $1.99.
2) Just another of the many reasons to participate in the St. Jude Auction. I think I sent out 11 merino sweaters of various sizes this year. I think one guy got 4 sweaters for $50. Thrift store made some money, St. Jude got a donation and the guy got great base layers for cheap. Everybody wins.
3) As others said above, Camofire and Sierra Trading Post both have good prices occasionally on merino pieces. You just gotta buy when you see it regardless of time of year.
Whatever you do, don't spend any money on First Lite Red Desert boxers if you will be doing a lot of hiking or pack-in hunts. They disintegrate at the seams. Holes appear within just a couple days' use. They are probably fine for stand hunting for whitetails, but they are unk and wasted weight for backcountry elk or any mountain hunting.