While I do loathe the current fire dust prices, and personally agree with the idea that it might be expensive, I do understand why this system was most likely put in place/implemented.
For the vast majority of my time playing on the server (before fire dust was created), it encouraged me to explore areas of the server I'd never have been to if it was always in the game. It encouraged me to grind out for a broom and try to make it so that I could traverse the world easier. However, when fire dust was implemented, I switched to just constantly fast traveling and barely exploring at all. While this may not be the case for everyone, I do know that I barely knew half of the world's layout due to the instant skip of exploration. It limited me from wanting to look around and getting to know the world because it was just too accessible and easily obtained and removed any desire I had to purchase things such as brooms.
While the 20g price for single fire dust might be a bit too much in my opinion, I feel like putting a mental restraint on people attempting to breeze through locations and travel might be needed. Do I want to fly for 30 minutes across England? Absolutely not. But do I support expensive fire dust? To an extent, yes.
In conclusion, I mainly feel as though the intent of this is to encourage players to use other means of travel to get from point A to point B. The best broom is extremely expensive, most likely due to inflation, and if fire dust was cheap, you'd never have an incentive to try to obtain it, unless you are a collector or wanted an extra 3 m/s for whatever reason, seeing as how in this scenario, there'd be no point in flying a 48k broom across the map if you could purchase all the fire dust you needed in under 1000g. Along with this, the current situation results in players straying away from dependency on the system to be able to explore the world, as opposed to heavily banking all travel on a single means of travel.
To be honest, fire dust does need to change in prices, but I feel as though five gold for one fire dust is extremely drastic. We already know that you can obtain 75k gold just from quests alone, and the economy will most likely have some inflation happening, if not already. That being said, this is just my opinion, so it should be taken with a grain of salt. Thank you!