There are a few factors that play into the high expense of some items. For something like Astrals you are paying the high price because of what level they are. Sure you could have Electrum Rings that give only 5 MP less... but 30 levels later. It's the point of so much of a stat on so low of a piece of gear. These are far and away the best MP piece for your ring slots at that level and for many to come.

Why else are they so high? Because people will pay 1.5M for them... that's why. And why are people willing to pay such high prices? It's one of two reasons.

#1 They buy gil and if it only took them a few clicks and 5 minutes on IGE to buy the gil they don't mind spending it so frivilously.

#2 1.5M is a fairly insubstantial amount of gil to the person buying the ring.

I'd say the rising cost of such items is more of a result of #1. What will happen is someone will list an item for more than it's usual rate of sale and someone else just MUST have said item immediately. Only a gilbuyer (or someone painfully rich) would spend 500k-1M higher than the last going price on an item. Over Christmas when inflation went totally whack and Astrals were up from 1M to 3.5M on my server you could've gone to any gilbuying website and noticed that prices had fallen to a record low for their Christmas Sale. Gil cost nothing to buy these crappy companies so everyone that likes to cheat bought hoards of it and spent it without a care in the world. They just fed their need for instant gratification with reckless abandon.

As for #2 it might seem unbelievable to people earlier in the game that someone could casually chunk 1M or more at a ring without much though but I fit into that boat. People wonder why I would buy HQ2s of certain items (like level 14 stat rings) and it's simply because paying 50k more for 1 DEF and a blue box isn't an issue to me. Such amounts of money are mere pocket change that I could make in minutes. I could make enough to buy an Astral in a little over and hour. However, I still have to work for my gil so while I might be willing to pay 10k or so higher than the current going rate on consumables I will not pay 500k+ over the going rate to price gougers.

For most non-crafted items the above is what leads to the high prices but for HQ of crafted items there's a whole other reason. I'd rather not get into it here as I've covered what makes something like the Astral so expensive and it would turn into a long rant.