Bellow are a few threads where Marriott point valuations are disussed:
http://www.flyertalk.com/forum/marri...rd-points.html
http://www.flyertalk.com/forum/marri...tt-points.html
http://www.flyertalk.com/forum/marri...onversion.html
Note that most of the posts in these threads are before Marriott's big point devaluation earlier this year, so the actual valuations may have changed a bit. But you can see how different people come up with their valuations. From there, you can pick and choose what fits you the best.
Personally, I'm not sure there is a hard and fast "answer" to your question. For quite a few people, 15K Marriott points isn't worth a $300 cash outlay. But what if you need that 15K get just enough points for that 25th Anniversary trip/Honeymoon/etc? And if you have no other ways to get those last few points, then this may be your best bet and can save you many thousands of dollars over paying cash for your trip.
Another situation might be that you have been wanting to take your SO on nice little romantic weekend trip to Niagra Falls? If so, then you can solve two problems with one trip and the $300 may be money well spent.
Of course if you are only going to check-in to a local FFI, leave and comeback to checkout 3 days later just so you have the points. Maybe it isn't worth it.
The moral of the story is that the answer "depends" on you, your situation and your desires.