Ours is at playacar palace and is going to run about $20k too! We're having 70 guests and the (very rough/rounded) breakdown is:
Engagement Pics in our hometown (splurge): $500
Our trip (10 nights, 2 hotels-Playacar Palace and Live Aqua in Cancun): $5k (this might wind up being less depending on how our package's comp nights work)
Invites/thank you's/stamps: $1k
Favors: $700 (we're doing sunglasses and tote bags for everyone)
Flowers: $1k (bridal party/bride/groom/family corsages & bouts/etc. + reception table centerpieces which will be flowers and candles)
Wedding Package: $1500 (nautical)
Extras for seating over 30 ppl: $250
Groom Suit: $150 (guys have it so easy!)
Hair/Makeup/spa time: $500
Some of these things I rounded up on, so we're still under $19k. I figure I'll leave another 1-2 thousand as a buffer for some random expenses I'm not thinking of (like tips, setup and extra d©cor if we decide to splurge beyond centerpieces).
It's a lot but for 70 guests + a 10 day luxury vacation/honeymoon included, I think it's a great way to go It's definitely not "free" though, like I think some of guests get the idea it is for us...