Ukraine is not focused on land, they are focusing in asymmetric war. It is not possible, as of now, to hold all the land at no-matter-the-price, but it is possible to make it economically and politically not worth it. This is how smaller countries defend themselves - not only Ukraine.
I recommend you read a recent article in Foreign Affairs:
Putin Will Never Give Up in Ukraine.
The article itself is interesting and refutes your favorite thesis that Russia is supposedly incurring exorbitant costs and therefore the war will soon end. This strategy is wrong, it is not about money.