What is the value of an eagle quarter with an M mintmark?

The U.S. has never had an "M" mint mark. You're looking at the initials of the designer, Hermon A MacNeil.

The actual mint mark location is to the left of the date. Because you didn't mention a D or S I'm assuming the space is blank, meaning the coin was struck in Philadelphia. I'm also not sure what you mean by "good" condition. Numismatically, "Good" (capital G) actually refers to a coin that's quite worn. Moderate wear is described as Fine or Very Fine.

Retail prices as of 06/2008 for a 1927 Phila quarter are $5 in Good (= worn) condition, $7 in Fine, and $12 in Very Fine.