I was looking at ebay at explosion proof fixtures, and they're pretty expensive... Why is this?

Because to genuinely guarantee the proofing, the manufacturer has to do quite extensive testing on the design and every major component modification. And this testing is bloody expensive (we are talking about Meg $$ money). And these money have to come from somewhere.
Plus then during production, they are quite tied to the certified design, every modification (e.g. switch to other ballast model or socket supplier) means that modification needs to be reevaluated in how it is affecting the proofing, so generally it is avoided, what again cost a lot of money (the component vendor knows you can not that easy to move to some other one, so may pump up the prices).

Of course, cheepeese clones could be offered for fraction cost, but first when they are really 100% copies, they face intellectual property infringement problems, and when they are modified, without the testing you have no idea, how these will perform in the terms of the required robustness. Plus even when the copied parts seems to be the same, even use the same material, the manufacturing method may differ. And that means the set of possible manufacturing imperfections will differ too, so there is high risk of defects compromising the required proofing (the original design includes analysis of the possible manufacturing defects and their influence on the product safety, you can not copy that from a finished product.).

I would also say the production volume is probably much lower than with general use fixtures.


Every other explosion proof electric stuff is as expensive too like switches and junction boxes without even mention things like heaters and blowers.