Hello, and welcome to the HUBB community.
There is an entire sub-forum dedicated to paperwork issues that arise when you are riding in a different country or on a different continent - it is this sub-forum:
Trip Paperwork.
In that sub-forum, the second post from the top is entitled "
Here is where you get the cheap Green Card insurance". That post started life back in 2006, and as of today has 297 posts in it, so don't start reading it from the beginning or you will be too old to keep a motorcycle upright by the time you reach the end of it.
Instead, start reading at about
post #284, and read from there to the end. That should provide you with a very good understanding of how things work.
There are a variety of places that you can buy "basic liability only" (in other words, 'Green Card') insurance from. The policy and the coverage is identical everywhere, only the price varies. So think of it like you think of buying gasoline - find the lowest price and then buy it there.
Some of the places you can get it from are listed from
post #284 onwards.
Be aware that it is next to impossible to buy any kind of insurance for a North American moto other than basic minimum third party liability - in other words, you won't be able to buy collision, or theft, or comprehensive coverage, just third party liability. That massive price you mentioned ($2,900) is probably from one of the very rare providers of comprehensive insurance.
The monthly price of the basic liability coverage varies depending on what country you buy it in - the least expensive country to buy it in that I know of is Bulgaria, where it costs about €70 (US$ 70) a month.
Michael