Ah, you entered "900,000" rather than "900000". Normally that wouldn't be an issue (it would just do nothing*), but due to the way I implemented
this feature, non-numbers (like "900,000") got silently converted into 0 (a number) and then the system happily stocked your item for zero gold. Anyways, I've fixed this so that the only way to stock something for zero gold is to actually enter 0.
* Theoretically, it could just strip out the comma and use the number like one would expect, but it doesn't at the moment.