I was just under 6,000 points short of paying for a 4 day stay. Paying the SPG rate (better than Hotel comparison sites) would of been about US$254 for the additional nights. Instead purchased US$136.50 of additional points to cover those nights (US 2.275 c per point).
Considering how hard it is to purchase SPG points privately, looked like a good alternative.
Edit - Did this on my mobile so didn't end up getting cashback, Doh! Up-to 6.5%!
I read SPC and though I could get rewards for my baked bean breakfasts. :(