What are the most affordable options for replacing water-damaged bottom window panels sized W1040mm x H725mm?

No worries. Now that we can see what you have… woof! This window has seen better days. It looks like a single pane double hung or is it a single hung window? That window sash is beyond hope, and I’m guessing the frame is not much better. If the frame is solid, no rot, not spongy I’d look fir a new lower sash. But in guessing the whiting, frame and windows themselves are all bad in need or heavy maintenance or repair. Many times in older homes the windows were built on sight, so there is not standard size. That kind of mill work gets expensive, enough so, it may be cheaper and in the long run to just replace the enter window with a new modern assembly. Even a cheap modern window, will be better that want you have. Next question, is this a historical home, one that is on a register? If so you maybe subject to limitations as to what you can do. If it’s not, the only limits is your budget. What’s the shape of the window frame? Is it solid, or in need of repairs, replacement as well? But from what I’m looking at here, I’d be thinking about replacing the whole window, frame and all. I don’t know about your area, but where live, to pay someone to remove and old window, clean up the framing and replace with a new window, your looking at $700 to $1000 per window, ballpark price. And depending on availability and contractors as much as 2 months lead time. Good luck.

Adelaide? Hmm… If you cover the plane tickets and materials, I might be up for it. 😎 I’m way up north near the Edge of the Earth, basically on the opposite side of the world from you. 😂 I estimated the price based on quotes for my own windows. Assuming the frame is fine, or at least fixable, replacing this sash shouldn’t be too bad. Maybe around half the cost of new, between $300 and $500, just a rough guess. ‘t you have a window repair person or a millwork shop nearby? They might be able to reuse your hardware and glass. Once you have the sash out, take it to their shop and say, “I want one like this.” Put a piece of plywood in the opening to keep the elements out until you get a replacement. I’m not sure, but this method could be more cost-effective. If they have the original window, they won’t have to think about it; they can use it as a template to rebuild or replace yours. Good luck!