Just a slab of wood, and for wire brad nails! Cost: $0 ! Originally I used M3 screws for my projects but I find wire brads work great and the PCB just slides over top if you need to add/remove the board for some reason. Originally it was centered on the wood, but my PWM board arrived so I remodeled a bit. Drilled a hole cross-ways through it, and also longitudinally for a little wire management. The PWM board has a separate 5v rail for a 4A power supply (not pictured) to power up to 16 servos, so the power runs under the Pi A+ and pops up in front of the power terminal. I2C, 3.3v and ground wires run through the middle as a sort of wire management system.
At some point (next time I go to Radio Shack) I'll pick up a power switch to wire in to the 5V rail. Servos are strong enough to tear themselves apart or burn themselves up, so it's handy to have a switch ready to cut the power when you come across unintended bugs in your servo code!
Cheaper and faster than 3D printing something, and keeps the breakout board from skittering all over the workbench while prototyping my robotics project!