Late reply, but for upper 20's and 30's temps on a 15 mile ride, all I would need is normal socks and shoes and a pair of Performance Neoprene Booties. Right now they are on sale for $20 (50%off) These are quite a bit thicker than other booties I have seen or tried. Mid 20s and lower I add a layer of Performance Neoprene Socks on over top of regular socks. That gets me down to being comfortable in single digits on my 10 mile commute.

Also.. I wear a same model pair of shoes that is 2 sizes larger (44 vs 42) to allow for the extra bulk. Shoes that are too snug with extra thick socks will cut circulation and the socks will do more harm than good.

It also depends on how hard you are riding (ie.. avg. heart rate and blood flow). If I get cold I go faster. If I can't go faster due to weather/dark/etc. I make sure I overdress a bit.