Well, you're in luck if you've got some serious cash laying around -- 'cause a card featuring the two soccer legend's signatures just hit the auction block ... and it's an INSANE piece!

The Leaf Superlative Sports "Superlative Trailblazers" card features a picture of Ronaldo on the top half, paired with a piece of one of his white Real Madrid jerseys and his slick blue signature.

On the bottom half, Messi's autograph is accompanied by a swatch of one of his sky-blue Barcelona kits.

In the bottom left corner, meanwhile, you can see the card is numbered "1/1" ... meaning it's the only one of its kind in existence!!

As for the back of the card, Leaf Trading Cards guaranteed its authenticity there with a huge "congratulations" note.

Bidding for the item at Goldin Auctions is set to end this weekend ... and our sources say the card could sell for around $20,000!!!

Not a bad price at all considering Messi's now moving on to a new team and cards with his Barcelona swag will become even rarer.