Here's a little context for comparing the various versions of the Canon 70-200mm "L" lens series:

• EF 70-200mm f/2.8L was introduced in March 1995. This design is almost 25 years old!
• EF 70-200mm f/4L was introduced in September 1999. The design is almost 20 years old.
• EF 70-200mm f/2.8L IS was introduced in September 2001, over 17 years ago.
• EF 70-200mm f/4L IS was introduced in November 2006. That's over a decade ago. It was replaced in Canon's lineup by the EF 70-200mm f/4L IS II, a lens with a new optical formula, in 2018.
• EF 70-200mm f/2.8L IS II was introduced in April 2010. That's over nine years. It was replaced by the EF 70-200mm f/2.8L IS III in 2018, but the "III" has the exact same optical formula as the "II", other than some updated lens coatings that doo reduce flare significantly in certain lighting situations..

Some people think these are all pretty much the same lens with various apertures and with or without IS "added". They are not. Each lens is an entirely independent design and reflects the state of materials and lens design technology available at the time they were released. The EF 70-200mm f/4L IS, for instance, has four more elements spread over two more groups than the EF 70-200mm f/4L.

Some of the popular alternatives to the Canon 70-200mm lenses include:

• Tamron SP AF70-200mm f/2.8 Di LD (IF) Macro was introduced in 2003.
• Sigma 70-200mm f/2.8 EX DG HSM II Macro was introduced in 2001
• Sigma 70-200mm f/2.8 APO EX DG OS HSM was introduced in 2010.
• Tamron SP AF70-200mm f/2.8 Di VC USD was introduced in 2012.

As you have already noted in your question, there is a wide range in the pricing between these various lenses. Which one is right for you depends on several things such as your budget, intended use, and intended viewing size and audience for the photos you produce with it. For highly competitive professionals where technical image quality can sometimes determine the difference between you and your competition in the eyes of your potential clients, the price of top notch lenses like the EF 70-200mm f/2.8L IS II is just the cost of doing business. For many photographers whose livelihood does not depend on the technical image quality of their photos, these other lenses are also very good options.

If you are primarily shooting out of doors in bright light, the money you save on an f/4 variant can be used for other things. If you are shooting static or slow moving subjects handheld in low light, or you also plan to shoot video, you need a form of image stabilization. If the subjects are moving fast enough that you need a faster shutter speed than 1/focal length then f/2.8 becomes indispensable. There's a huge difference between 1/250sec @ f/4 and 1/500sec @ f/2.8 when trying to freeze the action in low light.

I own the EF 70-200mm f/2.8L IS II. It is the best zoom lens I have ever used. I had to save for quite a while to be able to buy it. Many meals that could have been eaten in restaurants were cooked at home. Many other things I wanted were put on the back burner. The cost of this lens was totally forgotten when I looked at the first images I shot with it. It is worth every penny I paid for it. I consider it some of the best money I have ever spent on anything. Whether that is true for you depends on what you need and expect out of a 70-200mm lens, and how much you are willing to pay for it.