There are a variety of issues that separate binoculars in one price range from binoculars in another. For example, assuming roof prism binos, at the $100 price point and under we have bins like the...

Bushnell Powerview
Nikon Sporter I/Buckmaster
Tasco World Class
Bushnell H20

These bins may be waterproof but are more than likely just water repellent. They may also only be "fullycoated or just plain "multicoated" in terms of the lenses. This reduces brightness, contrast, resolution, etc...

At the $200 price point we have binoculars like the....

Bushnell Excursion
Eagle Optics Denali
Pentax DCF-HS

These binoculars have at least multicoated optics and have phase-coated prisms. This makes them siginficantly brighter with a sharper image. Some models might also have a slightly wider field of view. The glasses may also be a bit lighter overall. Durability will, ofcourse, increase slightly as you pay more money within product lines from the same company.

At the $300 price point you start getting into the bins that give you great performance for the price. Bins included in this price range include....

Nikon Monarch
Bushnell Legend
Wind River Cascade and Olympic
Pentax DCF-HR II
Cabelas XTs
Minox BD BR or BD BL

These binoculars are all fully multicoated, phase coated, waterproof roof prism binos that offer excellent image quality with two or three characteristics that separate them from the $1000+ models

From $400-$800 you do not really see that much of a difference in terms of quality or design. These binoculars are a smidge better in terms of one or two characteristics than the $300 models but, IMO, are not worththe doubling in costunless you are trying to get the absolute highest quality instrument within that particular budget.Binoculars in this range include....

Pentax DCF-SP
Minox HG
Kahles
Bushnell Discoverer (original price)

Between $900 and $1000 you havea tier of binoculars that is possibly the best from their respective company but still falls just a bit short of the absolute best roof prisms available for one reason or another optically or mechanically. These binoculars would include.....

Bushnell Elite
Meopta Meostar
Zeiss Conquest

Between $1000 and $1300 you have some of the best roof prism glasses ever produced. At one time they were the premier roof prism glasses on the market however subsequent improvements in prism coatings or a reduction in weight due to construction materials has bumped them just a bit from the "latest and greatest" status. Without comparing them extensively to their slightly more expensive counterparts you would never know that there could be a better glass out there. These bins typically have full life time warranties, close to the latest advances in coatings and excellent build quality. The quality control at this price point is also much better than that of the bins in the $300-$800 price range. Bins in this range would include.....

Leica Trinovids
Swarovski SLCs
Nikon Premier LXLs

Lastly we have the most expensive roof prisms on the market...$1300+. These three binocular lines represent the highest quality level and latest technological advances in just about every area. There is currently some discussion on whether or not they are actually "better" than the models just mentioned so take it with a grain of salt. Bins in this range include....

Swarovski EL
Leica Ultravid
Zeiss FL.

To summarize, you pay more for

1. Better durability
2. Better coatings on the prism and lenses
3. Waterproofing
4. Warranty
5. Optical characterisitcs such as a wider field of view, closer focus, less edge distortion, etc...

Hope this proves helpful.