Here is a general guide to the value of a 1942 nickel with an "S" above Monticello:
In good condition (well-worn, with some details visible): $1 - $5
In very good condition (some wear, but most details still visible): $5 - $10
In fine condition (minor wear, most details still sharp): $10 - $20
In extremely fine condition (very little wear, most details still sharp): $20 - $50
In uncirculated condition (no wear, original mint luster): $50+
The value of a 1942 nickel with an "S" above Monticello can also be influenced by its rarity. Some variations of the 1942 nickel with an "S" mint mark are more rare than others. For example, the 1942-S "Micro S" variety is a rare variety of the 1942 nickel with an "S" mint mark that has a very small "S" mint mark. The "Micro S" variety can command a higher value than other 1942 nickels with an "S" mint mark.
It's important to note that the value of a 1942 nickel with an "S" above Monticello can vary widely depending on the specific condition and rarity of the coin. To obtain an accurate value, it's best to have the coin evaluated by a professional numismatist or coin dealer who specializes in U.S. coinage.