TMZ pulled Team Kennedy's FEC filings ... and it shows the campaign shelled out more than $68,000 to Russell's production company for "Event Entertainment - Travel Expenses" after he appeared at a campaign event for RFK Jr. and his running mate Nicole Shanahan.

Russell performed at RFK Jr.'s event in May out in Nashville -- and it was called "Night of Comedy," alongside other comedians including Rob Schneider, Dave Landau, Jim Breuer, Katherine Blanford, Jeff Dye, and Mike Binder. Russell reportedly did a filmed set where he went after legacy media and scoffed at coronavirus mandates and vax requirements.

Now, if you're wondering why an RB appearance might be this damn expensive -- amounting to $68,417.96, specifically -- we've talked to sources who tell us the money covered not just Russell's travel expenses (like the docs indicate) but also ... touched other elements of his performance, including paying for the transportation of his crew, equipment, etc.

Mind you ... the guy is reported to be living in the UK these days, so that explains a lot.

Anyway, it's clear Russ didn't give Kennedy and co. a friendly discount -- even though they are pals and RFK Jr.'s been featured on RB's podcast "Stay Free Russell" a handful of times now.

Interestingly, a few weeks after the RFK campaign event ... the comedian sat down with RNC spokeswoman Elizabeth Pipko, where he encouraged Americans to vote for Donald Trump over Joe Biden. Granted -- he was asked about the hypothetical between just the two of them .. and he didn't outright endorse Trump here.

Then again ... he hasn't outright endorsed RFK Jr. either -- despite being cozy with him.

Nonetheless, RFK Jr. has nabbed a number of celebrity endorsements, including Alicia Silverstone, Kevin Spacey, Jenny McCarthy and Donnie Wahlberg, among others.