@Filene's Basement - although @H&M is a popular choice, if your looking for something a little more refined stop by Filenes basement on Newbury, they have an excellent selection at reasonable prices. Depending on what your looking for,
@Lord & Taylor also has some good coats that are sophisticated. If you want to spend extra go to Barney's in Copley for fashion forward coats. What I have done in the past is @Marshalls across from Filenes Basement... I found a Miss Sixty coat there that I loved for just under 70 dollars. Otherwise you can be patient and look online at gilt.com and hautelook.com, usually I find my designer coats at discount there.