Optics are key. If you can afford Zeiss optics, try and find a used one in good condition. There are some online sites with good deals. If you want new, try Nikon, their optics now are quite good and are cheaper than scopes of equivalent quality.
Cost will definitely increase with a bino version, so you can easily afford a better quality mono version than bino. Of course, with a bino, your eyes are totally relaxed, and as long as you can merge the two images, you'll find the views to be superior over the mono. Again, this is also dependent on the quality of optics when making such a comparison.
Then, you should also think about whether you'll be doing any CCD camera work, and so may need to consider those options.