Lenny and Larry's Complete Cookie. 400 calorie / 16g protein / 64g carb cookie that doesn't taste like cardboard. I usually cut one in half for a typical ride, a full one for a long day. They sell 'em at Trader Joe's for $1.69 each, so not very expensive. The choco-mint flavor is my favorite.
Nut butter filled Clif Bars are a close second.