I'm guessing that is a basement window. It looks like the sash is fixed. I would guess that the entire sill is probably soft and should be replaced. I would replace at least the sill plate. I would investigate the damage and see if the rot extends inside of the outside vertical edge of the sash. If you dont want to spend the money to pull the window and replicate the needed window parts and the rot does not extend inboard. You could cut the sill board flush to the sash and have a new sash extension cut on a taper so rain water runs away from the window sash. Using waterproof construction adhesive glue to the old sash edge and the brick and concrete surround, mechanically fasten to hold in place. Allow to cure then scrape window clean, caulk edges to brick, prime and paint. However, that is based upon a cursory review of the picture posted. Contractor costs, I can't address that, I typically do my own so not sure. Regards..