Floyd Mayweather's Birthday Cake Cost $3,000 ... Came With Crystals (Pics)

Floyd Mayweather even balls out on CAKE ... 'cause his birthday dessert was covered in crystals and cost $3,000 ... TMZ Sports has learned.

We spoke with the maestro behind the insane dessert, Bree Miller -- who says she spent 72 hours on the 6-tier cake.

Plus, Miller says she strung 12 feet of Swarovski crystals around the masterpiece ... because she knew Floyd's cake needed that Money Team swag.