This Manfrotto 501PL Compatible QR Adapter base has been around for a while, typically closer to the $35-$40 dollar price. Here's a recently listed item that is offering this kit (QR base + QR plate) for just $22 + FREE SHIPPING.
Pretty good deal if you needed the extra adapters for moving your cameras to a Jib, Shoulder Rig, Slider, etc. You can also throw one of these adapters over other (non-manfrotto) tripods heads, so that they will be Manfrotto 501PL compatible. And unlike the more expensive OEM Manfrotto 577, this base has two bubble levels, a standard 1/4-20 + 3/8 thread on the side of the plate (for mounting accessories), and more options to mount under the base.
While it's available (found here).