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Rotted bay window frame repair/replace

The bay window (sash deal) in our student rental is rotting, with the hole becoming invasive with risk of the window pane falling out I imagine. See photo's. I'm not sure what the best option is. ? possible to plug it up as a quick short term measure, or replace. And if replace... how much needs replacing. A whole new double glazed bay window with PVC framing would be ideal but how much?