It's free if you do it yourself.
Paying someone to write you an article can cost anywhere between $5 and $500 depending on content, length, research, quality etc. Submission service costs also varies widely. Best to do a bit of research on the topic so you don't get in over your head or have someone take advantage of you.
Free is the best way to start out because you can learn how much effort goes into crafting a solid article then submitting it to article directories. From there you can gauge what you value the work to be.