The PI market is hugely skewed because of stockpiles of planetary products that players were able to buy cheaply before the Tyrannis patch.
To understand the scope of this we need to look at the history of these commodities in Eve.
Eve was derived from old single player games like Elite. It inherited a trading system where you could buy, say, 1000 units of antibiotics in one system for 200 isk each then fly them to a system 10 jumps away where the game would buy them for 250 isk each. The basic purpose of this system was to provide a way for players to make easy isk although it was quite time-consuming and profit was low.
By the time I started playing in June 2009 this system had almost entirely fallen into disuse. This is because most of the game had been affected by inflation but this system was more or less untouched since the game's earliest days. So as time had passed these trips had become not worth making. 10 jumps for 50 000 isk profit simply wasn't worth doing - courier missions were almost the same but paid more.
Now the game had been constructed to occasionally reward players with these NPC goods as loot. They drop during missions and also were handed to players for courier missions which were not always completed allowing the goods to remain in the system. So for a few years these goods were supplied to players without it really be worth anyone's while to cart them off to a NPC trader to turn back into isk. Some would have been sold back of course but for most players we all had a small amount of this somewhere around in an item hangar or two that we never bothered to do anything with.
When Planetary Interaction was announced most people weren't very sure what it would involve but there was speculation that these NPC goods would be used. This meant that the handful of traders still hauling them to NPC vendors dried up almost completely because there was a chance that this very low value material might one day be worth something.
When PI was launched on Sisi people realised that this stuff was pure gold. They could buy a NPC's stock of Robotics and be guaranteed that it would later be worth more than the NPC sell price of 7000 isk.
So several months leading up to Tyrannis all NPC goods vendors sold out every day, mission loot planetary products were bought and stockpiled. In May for instance over a million units of Robotics was traded on nearly every day with a high of 3.2 million trades on 13th May. All of that headed for stockpiles.
When Tyrannis launched PI wasn't in yet but the goods made by PI could be reprocessed. As Letrange explained, this meant people could buy a structure, reprocess it to get p4 products. The p4s could be made into more expensive structures and sold back to the NPCs. CCP quickly removed the buy orders but not the sell orders and people frantically converted the p4s into structures anticipating CCP deliberately breaking them. This they did, those p4s generated by that loophole now only convert into Tritanium but not before yet another stockpile of structures was quickly and easily built up.
So let's conclude. We have years worth of NPC goods found as mission loot and purchased in the feeding frenzy just before Tyrannis. We have a spike in structure supply caused by the clumsy implementation of Tyrannis. We have an ongoing trickle of planetary products from missions: things like Robotics, Coolant etc all still occasionally drop when you blow up structures in missions. And we have a PI system which as yet doesn't really consume a vast amount of resources.
All this means that certain PI materials are oversupplied and will be oversupplied for some time.
To get a picture of the ratio of oversupply to consumption here's a very rough example. SBUs use 560 Construction Blocks each. In The Forge (approximately 60% of the Eve market) about 2 per day are traded. We can assume that most of the SBUs traded are consumed rather than re-traded (flipped, etc). Let's assume too that SBUs consume about 20% of planetary products used. So about 5600 Contruction Blocks per day will be consumed by the market.
Well over a million Construction Blocks have been traded most days going back a year. On 4th of July there were 2 095 311 Construction Blocks traded. Those trades were speculators selling to each other and new PI players testing the market as well as mission runners converting loot to isk. But those 2 million blocks is enough to keep the market supplied for over a year if my estimate of 5600 consumed per day is right. Even with no one making them for profit the supply of Construction Blocks from stockpile liquidation, noobs trying out PI and mission runners selling loot is likely to fully supply the market for years.
This means that it is unlikely that Construction Blocks will reach anything close to the natural level for a P2 item as determined by production costs. Anyone producing them using PI will be undercut by the many other sources of supply.
This will apply to every planetary product that existed in Eve as a NPC good before PI. Here's the list:
P0 - all new
P1 - Oxygen, Silicon, Water
P2 - Construction Blocks, Consumer Electronics, Coolant, Enriched Uranium, Fertiliser, Genetically Enhanced Livestock, Livestock, Mechanical Parts, Miniature Electronics, Polytextiles, Rocket Fuel, Silicate Glass, Superconductors, Synthetic Oil, Transmitter, Viral Agent,
(ie all except Biofuels, Microfibre Shielding, Nanites, Oxides, Polyramids, Supertensile Plastics, Test Cultures, Water-cooled CPU)
P3 - Guidance systems, High-Tech Transmitters, Planetary Vehicles, Robotics, Transcranial Microcontrollers, Ukomi Super Conductors
P4 - none
So all those P2 and P3 listed are things which should be cheap, will be kept cheap by over-supply, and are not worth producing when you can buy them cheaply instead.