How much is 1957 silver certificate K?

"K" is either a plate ID or part of the serial number. In either case it's not a determining factor in the bill's value. What matters is what letter if any appears next to the date. There may not be a letter, or a small A or B.

However, regardless of whether you have a 1957, 1957A, or 1957B bill, all of these are very common among collectors and retail for at most $1.50 in circulated condition.