If you could double your budget, you'd have a lot more choices. I'd go for a full sized or reverse porro prism model. Do you wear glasses? If so, your options will be even more limited.
If I only had $50, I'd try to find a used 7x35 or 8x42 porro that sold for $150-$250 originally.
Failing that, I'd go to a big-box department or sporting goods store and sort through their cheapest 7x35 and 8x40 porro prism offerings (Tasco, Bushnell) for a unit that I thought was usable. At this price level, there are a lot of assembly errors, poor optics, and designs with poor mechanical integrity (so the focus control is so sloppy that it doesn't really work well enough for birding). Problem is, many of the cheapies are in bubble packs these days, so you can't try them easily.
If you can spend a little more, consider Nikon Action 7x35 ($65), Bushnell Legacy 8x42 ($65), Leupold Yosemite 6x30 ($90), Leupold Yosemite 8x30 ($105). If small size is a concern and you don't mind a restricted field of view, a good reverse-porro in an 8x25 configuration could be very good, but all the ones I'd recommend would be $75 or more.
--AP