If you do go on your own (DIY), just make sure you order the right type of window... most come in a new construction and retrofit versions. The retrofit ones are designed to fit into framing per your measurements of the window openings.

Some government programs allow you to deduct a percentage of the labor cost as well, so that could factor into the overall cost. All it takes is one goof to really negate that DIY savings if the difference isn't substantial.