Farrah Abraham -- the most famous "Teen Mom" alum -- will have to dig deep into her pockets if she wants to board the "Teen Mom" Cruise ... which is somehow a real thing.
A spokesperson for the cruise tells us Farrah wasn't asked to be one of the main attractions for the fan event because they're aiming for a family-friendly environment. Instead they went with Maci Bookout, Amber Portwood and Kailyn Lowry.
Farrah's still welcome aboard if she has $734 for an inside stateroom or a top tier junior suite ... for $1,303.
As for the cruise ... it goes down next July on Royal Caribbean's Enchantment of the Seas, sailing out of Miami with stops in the Bahamas and Key West.
One giant catch -- you gotta be 21 to book the trip. At least there won't be any future teen moms conceiving at sea.