Quarter Dollar with a M mint mark?

"M" is the designer's initial (H. MacNeil). The mint mark on a standing Liberty quarter would be on the opposite pillar in a similar position. Blank = Phila., S = San Francisco, D = Denver.

Without information about its date it's not possible to value your coin. If the date is worn off (a common problem with SL quarters) it's only worth its metal value, about $2.50 as of 03/2009.