I would like to add a dust cover for one of the models I´m building. It´s an architectural model which will be given to the client later on and a dust cover would protect it as they wouldn´t have to dust it.
But I can´t afford plexiglas or glass sheets. I checked the price for plexiglas and it was in the region of 100 dollars per square metre.

Is there any way to build dust covers yourself? and what material could I use? The base is about 40x40 cm and it will be roughly 20cm high (with trees).

If you can't afford plexiglass or glass, you may be out of luck.

You could try sheets of clear styrene, but it would scratch very easily -- the first time someone dusted it, it might be ruined. It would also be pretty flimsy, so you'd have to make a frame to hold it.

I know glass doesn't cost that much, it does if you have somebody build you a wood base and all that, but if you do it yourself, you can do it for a few bucks.
Find a small fish tank, with a plastic frame, and use the frame as your mold, a cheap fish tank should cost 4 or 5 bucks, and get your glass cut to size, and get clear silicon to glue it together. Pop it out of the mold, and you have a clear glass dust cover, or use plexiglass. Most places will cut to order.
You can add a wood base if you want, and cut groves in it for the glass to set into and that way it will be air tight.
The thing to check is if the place you buy it from if they will cut it on a 45 degree angle's, so it will look a lot better.
I know even PPG cuts glass to size cause I picked up a whole semi load of cut glass in Illinois, and took to Michigan so they could make small fish tanks of a few different sizes. Hope this helps

There are VERY cheap frameless fish tanks made from glass available here in the UK. One of these simply inverted will do. Try a pet store. Buy one that will comfortably fit over your model, fit the base of your model to a new base that will fit the inverted tank.

Vinnie

Thanks for the advice. I´ll see if the fish tank idea works.