The few thousand copies is over the _entire_ lifetime of a book, not meant per year. Again, your College 101 book in Physics or Calculus will be used in large courses and thus sell a large volume, if it is being that used that is. There are also many competitors in the market, and yours might not see significant pickup.
So, let's suppose that you indeed sell 1,000 copies at $150 a piece. Your share is 6% of the net sales, so $8370. From this you need to pay services such as the person making the index, or pay royalty for pictures (yes, many publishing houses deduct this from the author's earnings).
Let's say your book has 700 pages, which is pretty normal volume size, and after you paid for additional services and expenses you are left with $7000. You are a good writer, so planning the book, writing it, creating examples, copy-editing and rewriting it, making reference solutions, etc. and you only spend on average 4h per page. You have made $2.50 / hour before tax. And you probably have a PhD in that topic.
No, almost nothing of the $200 actually goes to the author. A typical share goes like this: (concrete numbers for Germany) 50% of the sticker price goes to the book seller, 7% to tax, the remainder to printing, typesetting, logistics, the publishing house etc.
A typical author would get somewhere between 4-7% of the net sales. Most books, especially advanced science books, on the market (not your College 101, not NYT bestsellers) sell only in the order of hundreds, maybe a few thousand copies. The vast majority of books doesn't even earn the advance back. The reason why they are often not good, is that it takes months to write 500 pages of well-thought out, well-delivered material, and the incentive to do that for $5000 is relatively low.