Maginfication is roughly the ratio of the two lenses focal lengths. A reversed 50mm on a 100mm lens is roughly 2:1 (2x). 25mm on the 100mm is 4:1. 50mm on a 200mm lens would also be 4:1.
Reverse rings for close-up work are as old as interchangeable lenses themselves. I have a circa mid 1960's adapter for my Minolta film cameras. The current wrinkle is to mount them on the front of your lens using the filter threads.
You can always pick up manual focus fixed length lenses for relatively cheap. Orphan mounts like Canon FD, Minolta MD, Konica a few others have a lower resale value (unless it is a "cult" lens) since the adapters for them to be used on current DSLRs requires an optical element for infinity focus. This degrades image quality. In some cases you can get glassless adapters but then the lenses can only be used as a close-focus/macro. With the advent of mirrorless interchange-able lens camera bodies (4/3 and micro 4/3) these mounts have found new life and prices are going up accordingly. You might try buying problem lenses (sticky, oily aperture blades, etc.). As long as the optics are fine (no fungus, clouded lenses) or the filter ring damaged these should be perfectly usable.
A caution or two with using the reverse rings. Many of the modern lenses are using plastic for the mounts and lens bodies. The additional weight on the end of the lens might damage the plastic mount or helicoid of the lens. Manual focus lenses pre-early 80s are use metal entirely for the bodies. The rear-elements of your lens (optics, mechanical or electrical linkages) will also be exposed and vulnerable to damage. You might consider buying a cheap 3rd party end cap and cutting out the center.
There are always close-up lenses. These are relatively in-expensive. They attach to the front of you lens like any screw-on filter. While they do degrade image quality you can still get stunning images with them. Multiple element close-up lenses with much better image quality exists but they are rare and expensive.