Lil Nas X pled not guilty to the four felony charges against him ... and now he knows what it's going to cost to post bail.

Nas waived his arraignment and entered a not guilty plea Monday ... and a judge set his bail at $75,000 with some drug-related conditions.

Assuming the "Old Town Road" rapper makes bail, he's been ordered to not own, use, or process illegal narcotics. Nas would also have to enroll in an outpatient program.

That's interesting because when police arrested Nas ... cops initially thought he might be high on drugs. They took him to a hospital and later transferred him to jail.

TMZ broke the story ... Nas is facing years in prison after the L.A. County District Attorney charged him with 4 felonies ... including battery with injury on a police officer.