Which is the best lens that I can buy, why are some lenses of same focal length too expensive

A lot of people buy a camera then go about deciding on which lens(es) to buy.

Rather if you still in the process of buying a camera I would rather suggest you to first have some clarity on what your choice of genre (look at what you like to click on your phone)

A lot of times the choice of lenses can dictate which camera system can be the best fit for you. (need a in depth comparison of finer points in a lens to elaborate my point and will hence skip it out here)

Having said that, it's not the end of the world if you are a beginner / already have a cropped dSLR.

Typically if you are looking at photography as a profession, it's better to stick with Full Frame bodies & FX Lenses (for nikon) and the "L" series from Canon or even the "ART" series from Sigma (3rd Party Lenses)

The overall quality of a lens is determined by how it performs over the entire range of focal length and apertures. Basic / low end lenses significantly underperform at their operating boundaries while having acceptable results in the middle range (Ex. slow focus at maximum zoom, vignetting at wide angles, purple fringing, aberration etc)

For Beginners  I would suggest :
a 50mm f/1.8 Prime lens (this is a budget lens with can produce excellent results)
the newer 18-135 (canon) 18-140 (nikon) this is a very versatile lens in terms of focal length which almost eliminates the need to change lens for everyday shooting.
For someone interested in wildlife a 70-300 is a good starting lens (do not expect much performance out of it)
For someone looking at shooting wider landscapes a 10-18 or equivalent would be a good option that would not burn a hole in your pocket
(NOTE: some lenses mentioned above are for cropped bodies)

For someone more serious (or on full frame) :
A 24-70 or an 24-105 would be good starting point (24-70 f/2.8 is preferred for shooting people whereas a 24-105 is more for everyday use)
a 70-200 (f/4 or f/2. are excellent portrait lenses. Someone who cannot afford these can go in for primes in 85mm or 135mm
Do note Macro lenses produce very sharp images and capture a lot of detail (100 or 105mm) and may be too sharp for liking of a portrait photographer (remember you can still soften during post processing)

If you need information on lenses / extenders (or combination of these) or lenses for specific use cases do let me know, will elaborate further