I have been using this Big Skinny leather wallet every day for the past three years and it has held up excellently. Stupid thin, check. Leather, check. Under $40, check.
I have always liked Big Skinny. Their wallets are insanely slim, mainly because the inner walls are a very thin synthetic. I am currently rocking this one http://www.amazon.com/Big-Skinny-Slimline-Leather-Bi-Fold/dp/B0048F88CU which has a leather outside. They are built very well, and I have not had any problems with them (this is my second, the first one got stolen).