I've got a pair of Berghaus 'Argentium' synthetic long johns that I wear for winter walking, climbing, camping, caving etc. They're warm, comfortable, and seemingly indestructable. I think they're inexpensive compared to merino leggings.
Also Mountain Warehouse sometimes have their merino/synthetic mix thermal ski-type tops on sale - I got one this year and it's super warm, a good fit, and washes better than pure merino. It is a bit sweatier, though, and smellier than merino.