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).
Manfrotto Compatible Quick Release Adapter for 501PL Plate
I've lost a few myself. Once that little guy is gone, you've rendered your tripod quick release plate pretty much useless. You can order these replacement screws through some of the big retailers for over $10 dollars, or you can check out eBay for some cheaper prices. Problem is you've got to wait weeks for that over seas shipping, so might be something small to keep handy. Call it Preventative Maintenance..
1/4 x 20 Replacement Screw for Quick Release Plates
[Update] This one would work out as well, but doesn't look like the standard 'slotted' screw head, where a quarter comes in handy. http://www.bhphotovideo.com/c/product/302185-REG/Wimberley_SW_100_Extra_Screw_1_4_20.html