Decorative dormers range from $583 to $933, according to a 2013 report by Dryden Mutual Insurance. An average 10-by-15 shed dormer costs between $11,270 and $16,904, while the same size gable dormer ranges from $14,400 to $21,602.ut this extra bit of headroom comes with a high price tag, and dormer installation is not a DIY project. Plan to spend between $10,000 and $30,000 for your new dormer, depending on size and style.