SPG has a 20% off sale for buying Starpoint from their web site. The price is $14 USD for each 500 block of points, you must purchase in 500 mile blocks.
Please Note: Cost is $.035 per point. Purchase up to 20,000 per account per calendar year; points are applicable toward all Starpoint awards but do not count toward elite status.
I don’t think is great, but it does work in some cases. For example, the Sheraton Grande Tokyo Bay Hotel costs JPY 25,000 per night (approx $318 USD.) However, using the cash & points option, it’s 4,800 SPG points+ $90 USD.
Rather than paying full price for the reservation, buy 5,000 Starpoints at $140 USD + $90 = $230 USD total for the room, saving you $88 USD. You’ll still get a credit for the stay since SPG awards count as qualification stays/nights.