I've got a cheapo Winchester-branded green rubber armored 12x25 monocular. Works surprisingly well and has survived almost 10 years in rain, cold, heat, etc. The one Rasyad linked looks like a very nice monocular. The 42mm lens ought to make for a nice bright view. The little 25mm lens on my Winchester isn't the brightest, but it works well enough for me. Not bad for $20.
I prefer 8x as it is difficult to hold 10x and greater without a jiggly picture. One trick if all you can get is 10x is to put your bow on your foot and hold the mono/bino against the bow when viewing, using your bow like a monopod.
I really like monoculars too. Very compact, fits in a quiver or pocket. Great for hikes and wilderness trips too.
Im looking to upgrade, my 8x32 nikons just dont show me much at 3d distances. They are light, but not good for anything else. I was thinking bigger objective and more power. The idea of a mono sounds great, and I could use it with my non aiming eye. First time I tried binos it screwed me up big time. I have since figured out to guess yardage first, then look at the target. If I view the target first, it screws with my mental yardage estimation.
I have a vortex solo in 8x42. It's "ok" optically, but not very sharp around the edges. I kinda wish I went the bushnell that was around $150. Got great reviews. I do love the weight and portability though!
I have used a cheap monocular for 2 years now for 3 D. Distances only out to 25-30 yards. But, it works great. Light weight and only needs one hand. Can hold the bow in my left and monocular in my right. Easy, peasy.
If I was shooting field? I'd invest in a good monocular and be good to go.