Yes. More expensive Platinum weeks are worth a lot more points than a low season week, so the matching point buy can get a lot more expensive. There is a chart maintained by TUGgers that shows what different weeks are worth. Here is a link to that chart:
https://docs.google.com/spreadsheet...guovIlHjggivemuPWYpnkSaXg/edit#gid=1983378178
Using that chart, you can see how many points the week you may be looking at exchanges for (It also shows maintenance fees, so you can calculate the MF cost per point). Multiply the point value (rounded up to 250 point increments) by some estimate of the price Marriott will charge (list is $13+ per point, but for any purchase over about 2000 points, they seem to discount to under $12/point. For bigger purchases, you might even get under $11/point, but I'm not sure if they will go that low or not). Add to the points cost the cost of the resale week and you will have the total cost (closing costs will add a little more).
In the post above, TheTimeTraveler recommends you stay away from points entirely and focus on traditional weeks-based Interval International trading. If getting the absolute lowest price for week-long trips is your primary goal, that is probably very good advice. But points and weeks are not the same product. Points offer much greater flexibility -- the ability to vary your stay to lengths shorter or longer than 7-day increments, the ability to control which view category you want (exchangers, by contrast, get last choice in view category), the ability to choose which unit size -- Studio, 1BR, 2BR, or 3BR -- you want for any given trip, a more hotel-like reservation/booking system rather than the deposit/search waiting game inherent in weeks-based Interval International trading, etc. Traditional weeks, while much cheaper, are much less flexible than points. It's really dependent on what you are looking for, what you can afford, and how much flexibility you demand. I personally dislike the weeks-based trading process and like to book shorter stays often, so I'm a fan of points. Others on TUG get great results with trading and like the cost efficiency of pure resale weeks and find no need for points. You'll need to figure out which approach is best for you. I would recommend you not buy anything until you figure that out.
The point rental approach TheTimeTraveler mentions is also a lower cost option with no up front cost, but just be aware that if you pursue this approach and you do not own points of your own, you will be asking others to make points reservations on your behalf. They will control the reservation, but will add your name as an "additional guest" and you will pay that other owner for that booking, not Marriott. Some people are comfortable with these kinds of person-to-person transactions, but others of us are not. Again, you'll need to decide which camp you fall into.
On the other hand, if you already own points of your own, you can use point rentals to supplement your owned points. If you own points, you will have your own points account, and when you strike a deal with another owner to rent their excess points, Marriott will allow that owner to call up Marriott and transfer those points directly into YOUR account. That way, those points immediately become yours and you can make your own reservations and you are not beholding to that other owner to hold the reservation on your behalf.