My four kids have done the MAP test through our charter school. I personally think it is a better test than the state STAR testing which they also must participate in. It really is only effective if you have them tested in the fall and spring of each year and then track it over time. If your child is progressing well through school, I wouldn't pay for it or worry about it. If, however, you are worried about their progress in math or language arts, this can be a nice way to gauge their indiviual progress, as the scores are not grade dependent. The lexile score may also be helpful if there is any trouble in reading. Always keep in mind though, that familiarity with test taking may affect their first scores.
Hope that helps,