I need to know the difference between
 a cheap white cotton t-shirt and an expensive white cotton t-shirt
for my childs science project

Look closely at the seams; the type of machine-stitching and the
number of stitches per inch may be different on the two shirts.

With a micrometer, you can measure the thickness of the fabric.
Cheaper shirts are often thinner.

Hi chelseapooh,

You'll find a good start for the project here:

HOW TO BUY T-SHIRTS:
http://ohioline.osu.edu/ccues/tshirt.html

hummer