Is 172.85 a good price to pay for a stock photography picture. For a Poster?

Depends on what rights you gain from it.

If you now own the image and the copyright to it...then it's probably not a bad deal.

If you only have the right to make 1 poster from it...then no, probably not such a good deal.

How many posters are you going to produce, what size and where / in what market... these are the factors that determine the licence for your copyright and the value of the image for your purposes.

If it's a one off for your own purposes I'd rather buy a limited edition or a signed original as an investment...

So it's impossible to give you a definitive answer!

It is an excellent price if you plan to make $344 from it.