I recently had the pleasure of styling 2 different princess parties, both of which who used my cake idea in a slightly different way.
To create these simple cakes, start with an 8-10″ textured white cake from any grocery store. Both cakes used a textured, vertical ribbon design in buttercream frosting. For the topper, simple tiaras were used. My favorite sources for tiaras include Target, Dollar Tree, Oriental Trading and Birthday Express. Generally speaking, you can purchase a tiara for under $4. To finish the cake, simply wrap the base with 1 1/2-2 inch grosgrain ribbon and secure it using a straight pin in the back (just remember to remove when serving :)). For an added touch, lay a princess wand in front of the cake made from the same material as the topper.
To recreate a cake like this, expect to spend about $30.