Expensive bubble levels are quite accurate and display an independent level for each axis. While the cost is certainly low to manufacture, there is a higher cost to getting consistent quality control.

The accuracy of cheap bubble levels is not very accurate. They sometimes do not even have the bubbles parallel to the base of the hot-shoe! It seems like such a simple thing to make.

My daughter loves the bubbles so she used to take mine which I have 4 expensive ones for historical reasons (Dotline, Kaiser and 2 Jobu - which is a famous brand BTW), so I bought a lot of 10 for a ridiculous price for my daughter. She loves playing with them but none of them are correctly aligned on more than one axis.

The point is that those things - just like the levels inside cameras - have to be incredibly accurate otherwise they are useless. It is not good enough for them to be close because close can be done by eye. Without a level I get shots within 0.5° of accuracy. Any level less accurate than that is dead weight. The one on my Pentax K-7 and the Canon 7D were accurate to about 0.25° (although the specs on the 7D say otherwise) while I turned off the one on my K-5 because it was less accurate (about 0.75°).