50 Cent: It Ain't Cheap to Be This Jacked

Now we know how 50 Cent keeps his body so swoll -- he drops a fortune to keep a trainer by his side nearly 24/7.

In fact, according to docs filed in his bankruptcy case obtained by TMZ ... the rapper pays more than $3k a month in workout expenses. A big chunk of that -- $2,800 to be exact, goes to his trainer, Jay Cardiello.

Jay has talked about working with 50 before, saying he pretty much eats, sleeps and breathes with 50 ... since they train together 3 times everyday.

And get this, despite the fact 50 hasn't had a #1 hit on Billboard's Hot 100 since 2003 ... he still makes $36k per month in royalties.