No, seriously. Cinder blocks can be pretty awesome for a desk. They’re like a buck or two at home depot, they’re as solid as a rock, you can use your existing desk, and you could even cover them with wood or cloth if you really hated how they look. Lots of people have done this with pretty great success.
The main downside I can see for this approach is that you can’t really fine tune the height of the desk. If you want proper ergonomics, you’ll need to get your height set to within an inch or two. That’s something that cinder blocks probably can’t accomplish easily. If you’re one of the lucky people whose elbows line up with one of the discrete heights you can get by stacking cinder blocks + your desk height, then go for it! Otherwise, you may want to look for a more fine tuned option to protect your neck and back.