Marginal cost of window (framing) while in design phase
Where I live, 4x4 windows cost about $600 and another $300 for framing, header, etc. Trim around the opening varies from $100 - $200, depending on type of trim, stain or paint, etc. 

Framing is not what is expensive in a house. It’s the finishes. 

BTW, an “egress window” is required by Building Code in all bedrooms AND sleeping areas. It needs to open a certain amount and can’t be more than 42” - 44” or something like that off the floor. Check with Building Codes in your area.