Here is an incomplete list of why it is perfectly logical for epriton prices to have been dropping!
1. Walls made unwanted PVP action and associated losses less likely, so less demand from lost equipment there, which has an lowering effect on ALL mineral prices.
2. Walls made mining epriton much safer, causing a supply increase there versus demand, effect: lower epriton prices.
3. Recycle nerf from not too long ago made other ores then epriton less abundant trough recycling. And because all ores are used in a fixed proportion to each other, there is a relative drop in supply of these ores versus epriton. Meaning for a static market price, epriton bar other causes has to drop relative to these other ores.
4. Over time older players develop their own knowledge base and make certain things them selfs cheaper then current market prices. Or they make prototypes as those are not really abundant on the market (some prototypes are really cheap to make and are magnificently better then their counterparts). Result: drop in demand of these kind of modules!
5. Over time hardcore producers need less materials then before as their efficiency extensions are closing on to level 10! Again less material is needed, causing a downward effect on prices.
6. As with point 5, there are the miners that also see an increase in efficiency over time, causing minerals prices to go down over time.
What it all boils down to is that things get cheaper over time if there is not something that upsets this trend. Think of huge wars, a major influx of new players needing equipment and new toys (resource hogs) to please existing players.