Buying miles & points is typically best when those points are sold below or close to "The Points Guy" (TPG) valuation of the points considered for purchase. That said, there are plenty of cases where you can get outsized value from the points where you might still consider a purchase, especially if purchasing within limited time promos at the historically lowest prices (or tying the lowest) that points have been for a year or more.
Points Examples to Be Tight With on Purchases
United Example (United Miles Are Never Sold within 170% of the TPG valuation) Saving 82% on Business Class Flights via Points Purchase & Even Cheaper Than if Paying for Economy Flights in $
Delta: Points Example With Limited Niches on Purchases During Peak Promos
Amtrak: Viable Points Purchases During Peak Promos