Crystal Harris -- Hef's Ex-Fiancee Sells $90K Engagement Ring ... At 50% Discount

Deal of the century -- some mystery buyer just purchased the massive engagement ring Hugh Hefner gave to Crystal Harris ... for a fraction of the original $90,000 price tag.

Sources tell TMZ, the unidentified bidder won Crystal's auction this morning -- bidding $38,000 for the 3.39 carat circular-cut diamond. With taxes and fees, the final bill was $47,500.

Shockingly, the ring actually exceeded auctioneer expectations -- because it was only supposed to sell for $20 -$30k.

Hugh reportedly shelled out the full $90k for the ring when he proposed to Crystal less than a year ago -- but Crystal called the wedding off.