I have more monocs than I care to admit.
Some are made by cutting old independent-focus binoculars that are cleaned.
Monocs are even shakier under high power in a hand, so avoid higher powers
generally.
But...about new retail ones:
---At the low end:
---The Bear Grylls (Bushnell) 9x32 is amazing for its price ($30-40)
...it has a stiff focuser, though.
---The Brunton Echoes (7x18, $20) are good, but the focuser has
hysteresis so you can't get the sharpest image. (tried in shop)
---I've used a Minox MD 6x16 before. Excellent portability, easy use,
short near focus, and very good field width. A lot of fun. focus a little jumpy.
I got the Audubon 6x616s, but they are unfortunately not made anymore.
---Carson 7x32 X-View : $47, but not to be underestimated. It's a little big
for a monocular, but very bright and sharp, has a tripod mount, and
extremely close macro-focus (butterflies, hummingbird at the window, ants,
that piece of ancient pottery at the museum in a glass case....super for museums)
And it works well at dusk or in shade.
---Vortex Solo 8x25: ($9) A little jumpy but excellent size and sharpness.
---Vixen 6x16: ($99). This is a lot like the Minox but a little less cool and
much more exact focusing.
You can get a lot in a monocular for your limit.
You might be tempted to go for an "Alpha" for a little more
(Zeiss makes the ultimate spy-monoc at 5x10) but dropping aperature
to keep price down is a losing game. If you ever go super-premium
the price will have to be multiples of your limit. Monocs are for a quick
look and easy stashing, so your price limit will get you plenty.