I go by quality and design first, then price. If it's too expensive, I don't buy it. I say that if I would have to save for it, it's out of my price-range. I try to stay realistic.
I recently found fantastic basic cotton longsleeved t-shirts/undershirts at H&M for 11 €. I was tempted to buy at least 10, 'cause they seemed to be good quality, but I only bought two so I could check what happens when you wear and wash them. They hold up beautifully! I'm very sceptic about H&Ms quality in general, but you can find amazingly good quality if you have an eye for it!