Slim Thug’s key to staying rich is avoiding spending sprees on fancy clothes -- the Houston Boss Hogg admits he has no issue rocking discount threads ... even in da club.

Slim gave his fans a masterclass -- with more than a dash of homophobia, though -- as he chastised dudes who flock to clubs wearing the same expensive designer outfits.

In his post Thursday, he said he keeps it playa for just pennies, and proved it ... yanking the price tags of his top and shorts to reveal he'd dropped less than $20 for both.

Seems like price isn't his only issue -- he also mentioned gay designers make all those high-priced clothes. Basically, Slim thinks Dior and Gucci shirts make people look gay.

Um, WTH? 🤔

Hard to take his advice on saving cash, since he also bragged about dropping millions on jewelry to distract from his frugal fashion choices.

ST says he's seen many rappers go broke splurging on their wardrobes. Yeah, we're sure it's the clothing bill and not the bling.

Anyway, fans found Slim's message confusing ... considering the rapper's selling cargo pants for $75 on his own website!!!