Search often! Just this month I ordered a few Theory sweaters for under $50 ($51.41, technically) each and went to link them and they're back at $190 - $255 each. I'm not sure how or why this happens but sometimes different color/size combos of things can be on major sale, so don't forget to search for quality items too! I ordered a size up in one of the sweaters just because it was on sale, and it turns out I'm glad I did.
For cheaper items, I avoid ordering from Asian countries. The sizes run VERY small in my experience, even when going by the size charts.
Edit: As an example, these were 3 of the things I ordered and the prices they were Jan 7
vs the exact same items now: