This is probably the last time Starpoints are going for 35% off - you have until 20 July 2018 to purchase up to 30,000 Starpoints and receive a massive discount off the regular price. As you should already know, the Starwood Preferred Guest (SPG) program will be merging with Marriott Rewards and Ritz-Carlton Rewards in August 2018 so if you do take advantage of this promotion, the 30,000 Starpoints will be converted into 90,000 Marriott Rewards points in August.
Starpoints go at USD 35 per 1,000 block but in order to benefit from the 35% discount, you will have to purchase at least 5,000 Starpoints. If you have not had the chance to experience the Japan Airlines Business Class product for yourself, remember that 20,000 Starpoints will convert into 25,000 Alaska Airlines Mileage Plan miles which is sufficient for two intra-Asia Business Class flights - all for USD 455 (after 35% off)!
There are not many instances where I would advocate buying the Starpoints at 35% off if you are looking at redeeming them for a free night (unless you have done the math and worked out that it is indeed significantly cheaper to redeem). With that being said, I am personally tempted to take advantage of the 35% off promotion (since Marriott Rewards Points typically sell for a lot more) considering how this may very well be the final one.