REI had a sale on Merino base layers recently. Also, Merino.tech on Amazon is affordable, though I haven’t tried it myself. Keep an eye on sales, sometimes you can find great deals on premium items.
REI Co-op sales can be a goldmine for finding high-quality Merino wool base layers at a steep discount. Shopping off-season can increase your chances of finding these deals.
For used high-quality Merino base layers, Poshmark and eBay are worth checking. Also, Sierra often has discounted Smartwool and Icebreaker items.