A Holy Grail collection of Patrick Mahomes cards is up for sale ... and some of the pieces in the set are so rare and unique -- the asking price is a cool $7.5 MILLION!!!
The owners of the cards -- 29-year-old Robert DeArmitt Jr. and his wife, Heidi -- tell TMZ Sports the collection took about 3 years to amass ... and it all started as an investment project for the couple back in 2017.
According to Robert, he and Heidi saw Mahomes play often as Texas Tech fans .