Nether Reactor uses gold blocks

Not a bug.

i think the high price is to prevent users from building too many nether reactors. it's a mighty tool, so there has to be some kind of a limit to ensure a better game balance / experience.

maybe you can compare the high priced material to build the nether reactor with something like uranium in the real world.

If you're quick enough, you can mine the four gold blocks immediately after starting the reactor, before they turn to obsidian.