There are many versions of this cake out there (Made-In-the-Pan Chocolate Cake, Vinegar Cake), and it's as easy as any store bought cake mix. What makes it penny wise is the fact that it requires no eggs or milk - it's basically flour, sugar, cocoa, and water. Don't let the simplicity of the recipe turn you off - it's unbelievably rich and moist and only improves with age (I actually prefer it the next day).
My daughter decided to bake one up the other day as it was raining outside (hallelujah!) and we were all lazing about enjoying the cooler temperatures. She doubled the recipe to fill a 9x13 pan, but accidentally put in waaay to much cocoa powder. Never one to throw away perfectly good ingredients, we tripled the batch (6 times the original recipe!) and ended up with three 9x13 cakes. I generally have no problem finding a way to dispose of chocolate cake, but this was a little much even for me, so we sent one off to live with my mom and chocolate-loving step dad, and divided the second one between our two closest neighbors. We've almost polished off the third one, and it's still scrumptious almost 4 days later.
You can find the recipes for the cake and frosting here (we baked our doubled batch for about 45 minutes). The frosting recipe will give you more than enough for a single cake or batch of cupcakes, but we doubled it to cover all three of our large cakes.