I find that they're an opportunity to use up some fruit I've stashed in the freezer. This time, I used blueberries (natch), pulling them out of the freezer a day ahead of time, letting them thaw in the fridge, and tossing with about a tablespoon of sugar before starting to prepare the batter. Rhubarb is an excellent choice as well.
I'm ballparking that the cake here cost about $2.50, including cooking gas. You can shave about 50 to 75 cents off that cost by using non-organic ingredients.
Pre-heat oven to 375 F, and grease a small cake pan. Sift together the flours, baking powder, and salt. In a large bowl, beat the butter until light. Gradually add the sugar and cream until light. Add the egg and milk and mix well. Add the flour mixture and stir until smooth.
Scrape the batter into the cake pan and smooth it out. Toss the chopped fruit with sugar, if desired, and spread onto the top of the batter. Bake about 25 minutes. Let rest at least 20 minutes before serving.