I can agree with Mark on this.
I had no choice but to buy insurance in Mexico and Costa Rica, and apparently it is illegal to not have insurance for Chile and Argentina. But, as I went though many of those last two countries borders to get in and out of Ushuaia, I never actually was sold any, I never knew where to buy it and it was never mentioned by any official or copper.
So, rather than try and buy and pay money when it is not needed beforehand, buy as needed.
Colombia I had no real choice as we shipped from Panama and it was a requirement that we buy insurance in Cartegena. This was for 3 months, but if you fly into Bogota, you can bu it for one month only. As Mark points out he did without and got away with it
The same goes for Bolivia and Peru apparently, I read a few threads on this with some people claiming it was needed, and some actually designing an insurance certificate to cover themselves. The thing is, if there is no insistance at the border crossing, then I won't pay and chance my luck, the choice is yours.
Have a read of this site
here I used this extensively and although some of the info is dated, t gave pretty good concise advice for mst of the brders
Cheers
TS