Waka Flocka & Gucci Mane -- Footing GIANT Funeral Bill for Murdered Slim Dunkin

Waka Flocka Flame and Gucci Mane are throwing down big $$$ to cover funeral expenses for a fellow Atlanta rapper -- who was brutally gunned down last week -- lavishing their fallen comrade with elaborate memorial arrangements.

Rapper Slim Dunkin (center) -- real name Mario Hamilton -- was shot and killed Friday night inside an Atlanta recording studio ... after a verbal altercation with an unidentified man turned deadly.

Now, a rep for Waka and Gucci tells TMZ, the rappers are pooling their assets to ensure Slim is laid to rest in the best way possible ... with an elaborate funeral that includes a white horse-drawn carriage, massive white floral arrangements, and a white dove release ceremony.

We're told Slim's family will cover some funeral costs -- but Waka and Gucci are paying for the lion's share.