For deer stalking or wildlife observation, when you are 'glassing' the terrain looking for wildlife, the binocular is the superior tool.
Often though, when out and about on foot, or when paddling, you spot something with the mark 1 eyeball, and just want a closer look. Monoculars are very good for this.
There are loads of cheap 8x20 or 10x25 roof prism monoculars around. They are certainly an improvement on the naked eye, but to be honest they are not very good. A friend of mine was using one, and I passed him my Russian KOMZ 8x30 porroprism monocular and he was astonished at the difference. Of course, the Russian model is significantly chunkier.
Opticron make roof prism monoculars that are a cut above the cheap end. Recent (post 1990) Zeiss roof prism monoculars are phase coated and excellent, but pricey.
As garbo implies, for seriously high end optics you are really looking at binos. I recently bought a pair of Zeiss 8x42 Victory FLs, which although dreadfully expensive are worth every penny, and I wish I'd bought them years earlier.
Having said that, around £150-£200 will buy a pair of multi-coated, phase coated, waterproof, nitrogen filled Japanese or Chinese roof prism binos that, most of the time, will give you maybe 90% of the performance for 25% of the cost. The trouble is that in low light conditions, the difference becomes rather more marked.
