Depends on a lot of things, starting with "is it all original?" and then

- what gauge is it? In the pre-1960 years, the 28 gauge is the most rare, post-1960, the 16 gauge is most rare.
- what is the year of production?
- what grade is it? The grades and numbers of grades changed in 1960.
- Did it have any custom-order features, such as stock fit, engraving, monograms, etc?
- Does it have extra sets of barrels?
- Does it have a carrying case or luggage that came with the gun on the same order?

Collecting pre-64 Winchesters in the higher end of the output of Winchester gets more complicated than "year, caliber, finish." The shotguns, especially, have many more features or issues that affect their valuation than do the rifles.

Prices I've seen on Model 21's range from about $5K to over $25K.