The type of telephoto lenses that are available on the market to photographers today vary greatly in price and quality. Many consumer grade lenses offer a wide range of focal lengths in small and affordable packages that seem very alluring when comparing prices on lenses. One thing that should be noted however, when you do research for the type of telephoto lens to purchase, be sure to examine the specifications of the aperture on the lens. There is no direct comparison or ratio of focal length to price so you must be sure that you are getting the kind of lens that best suits the needs you have for it.
While a typical consumer grade telephoto lens can cost somewhere between $200 and $400 dollars, these types of low end telephotos only offer a maximum aperture range usually between ƒ/4.5 and ƒ/5.6. This of course is just fine if you are going to be taking all of your pictures in brightly lit situations only.
If you are a parent of a football player that plays their games in the evening times then you might want to consider that your needs are much different then a soccer mom that is taking photos only in the day time. As soon as you start looking at lenses that offer an aperture in the range of ƒ/2.8, the cost of the lens can soar into the thousands of dollars.
In photography, you usually will pay for the amount of light that a lens can soak up, not really the focal length of the lens that you are using.