C18 is developed version of C11 as C11 is developed version of C99.
Then I think
Price of C99 < Price of C11 < Price of C18 is right. I’m trying to learn C as my first programming language. And I thought Standard’s of C helps me a lot about my curiosity about C. I want to buy the standard. But what standard should I buy?
It seems reasonable to buy C18 as it’s the latest version of ANSI C. But why? Why C18 is $232.00 and C99 is $403.00? Does C18 encompass only revised section of C99? Or it’s for man who already buy C99? I feel so confused. And when I google 'ISO/IEC 9899:1999', I can easily see C99's pdf file at the first page. What is it? C99 is originally require money to access. Is it illegal document?
I’m trying to learn C as my first programming language.
Your first programming language as in "my first prgramming language ever" or "the one I will write most code with"? If you have no prior programming experience I suggest not to go into reference business right from the beginning, it will distract you from getting familiar with common language paradigms. There are really good tutorials and introductionary books* that will be much easier to digest. Most of the better books even have a language reference section.
About C specs / language specs in general:
C is more on the legacy side of evolution. Most things that were standardized with C11/C18 were already common as custom compiler extensions for many years. As a beginner most of these changes will not help you to get used to C. Secondly specs are written to represent a formal correct write down of a language, that is not quite easy to understand without some background in formal languages and/or compilers.
Furthermore you should check if your compiler(s) for your target platforms support a certain standard. There are many compilers that stopped progressing at C89 or C99. Nothing is more frustrating than learning about a useful pattern that simply does not work because of an outdated compiler. Also at a certain level of C programming a good compiler reference will become more important than the C reference itself due to many custom extensions and switches.
About books vs. PDFs: Books will not always follow the "oh its old, thus less pricey" rule. If an edition is almost sold out it gets rare and the price might go up (eventually leading to a new edition release if demand is still high). Most of the C specs are in public as PDFs, they are not illegal.
[*] Did not want to advertise here, I am sure you will find lots of good books with an internet search.