Like most Magic: The Gathering cards, the value of Relentless Rats depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Secret Lair Drop)
1 Gonti, Lord of Luxury
The Crown Zenith GG card is probably the most popular right now, but the Deos set one from 2004 is a nostalgic pick worth tracking down
In this section, the main section of a set, one can find the standard cards, the holographic rares, and the standard EX if it's the Black & White or XY era, the GX if its the Sun & Moon era, and the V if it's the Sword & Shield era
Putting together a Near Mint to Mint set can be a particularly daunting task as many of these cards were not carefully preserved early on
It's currently banned in Historic and Pioneer, but it still provides protection from removal and counterspells in Modern