What is the cheapest most basic Microsoft software you should buy if you just want the Microsoft Word feature for your Apple Mac laptop computer?

TextEdit is plenty for you, you most likely won't need anything else. If you do, DO NOT download NeoOffice. It is complete garbage. Use it for a few days and you'll know what I mean. I would download Lotus Symphony, which is actually much more powerful than Word, and free.

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The previous answer is not entirely correct. Text Edit cannot do 95% of what Word can do. It is more like 15%. If all you want to do is type, it will suffice. However, if you need any more functionality, I might suggest OpenOffice or Pages (made by Apple). Open Office is open source and free to download and use. Pages is part of iWork which can be purchased from the Apple Store for $79 and includes some powerful software. By the way, both OpenOffice and Pages will open and save to Word format.

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The best option for you would be to purchase the student/teacher edition which is the lowest cost package. That does require that you have at least OSX installed on the computer. It also allows you to install on up to 3 computers.
You may have trouble buying an educational version if you are not a student or teacher. Also, unless you specifically want Microsoft Word, you could use a program that is compatible with MS Word's .doc files. One such program is Neo Office. It is open source software and is totally free to use; it can be found through the link to the right. There you will also find the system requirements. If you do specifically want MS Word, and you can't get the Student/Teacher edition you can purchase Word on its own, but it is pretty expensive ($239 USD)
However, it's worth noting that TextEdit - included free with Mac OS X - can do 95% of what Microsoft Word can do, including reading and writing Microsoft Word formats. It should be more than enough for most people, so try using TextEdit before you spend hundreds of dollars on software you may not need.

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OpenOffice.org is a free and open-source office suite. It has an excellent text editor called Writer (it has a layout similar to Microsoft Word 2003, but it looks nicer), and it is absolutely free: http://www.openoffice.org/. It can read and write in .doc and .docx file formats, and OpenOffice Writer it is an excellent (and free) Microsoft Word alternative.