Lots of folks have no idea about what it takes and how much even the fabric costs. If she is someone that means something to you . . you might want to offer her to make it at cost of the fabric (ie she pays for the fabric, batting and backing) otherwise add up the cost of the fabric - add in something for your time and say in your "sweetest voice" ... "I am so pleased you like my work, but for me to create the table runner that you would like, the fabric will be __ and my time would be __...". Remember, that if you do this for $12 then you will be opening yourself up to a barrage of requests.
Good Luck!!