Quantifying a good purchase with anything is often hard; objectively; subjectively - How does one do it?
When it comes to video games of any genre it’s often a perplexing notion of ambiguous design to even attempt to judge what creates a good purchase.
So the closest i can come to creating a valid measurement of video game purchasing is the time factor. How much time you spend playing it shortly after purchase ratioed out to the price spent on said title.
That being said; Space Engineers is perhaps one of the best purchases i’ve ever made.
I purchased the game on steam for 15.00USD (Currently it is 19.99) - Almost a year ago.
In that time i’ve put in about 250 hours; i would consider that a good purchase by any measurement in my personal realm of concerns.