It varies - you can get cheap white t-shirts (not XXL size) and a one color, one sided logo for around $2 or $3, if you make a lot. Average is around $4 for a couple hundreds. The problem is how many people want a white t-shirt. Not me, and not many others.
Heavy 50-50 colored t-shirts, with a mixture that last about 20-30 washes can cost around $5 - $6, two sided sometimes.
But yes, it matters on the type of clothing you get. That cost can go from $1-$2 for the cheap stuff to $20 or more for the more expensive stuff. Quality does matter.