I don't see why that should be expensive/difficult. Ebooks are mostly small files. It would be hard to ramp up a gigabyte unless you end up with image laden items such as pdfs.
Synching can be an issue. I had a one of the early kindles, and it was fine until I hit a few hundred items. It would re-index and be completely unresponsive for 10minutes at a go. That could have been done cloud side. In the end I decided I needed to purge loads of documents/titles to get it useful again. But accidentally sat on it. So game over. Moved to a simple Nook and SDCard loads.
Synching can be an issue. I had a one of the early kindles, and it was fine until I hit a few hundred items. It would re-index and be completely unresponsive for 10minutes at a go. That could have been done cloud side. In the end I decided I needed to purge loads of documents/titles to get it useful again. But accidentally sat on it. So game over. Moved to a simple Nook and SDCard loads.