The High Ground, and Air to Air combat is the key.
Okay here's my take on all of this and it's really a plausible explanation..
First off when the Sleipnirs were first introduced, there was one thing completely missing from this equation, and in fact that item wwas basically sleipner mounted weapons (Like the Fire of Hel). Because everyone was crafting them and people wanted them (High supply low demand), there was a bottoming out of price and people could afford them..
But then someone decided to throw into the mix.. weapons for use in PVP areas (Most notably PVP4), and suddenly combat has become now 3 Dimensional not only on the ground but now in the air.(Yes for those who own sleipnirs, and Gungnirs, you can kill anyone below you, but if someone else has an aerial vehicle and you don't have weapons on them, then you are a sitting duck so to speak).
So what's been happening? Well folks who are Gungnir copter and Sleip Vtol jockeys are mounting weapons on their vehicles and flying into PVP 4 with impunity and are blowing each other out of the sky.. (Conversely those who are foolish enough to engage a ground target, don't realize that if they're low enough to shoot a target on the ground, then that ground based unit can shoot back (ever hear of Rocket launchers?)
so what is now happening is that folks are going into PVP loaded for bear, flying around and blowing everything either out of the skies or are 'ground pounding' the hapless miners/pker's below who can't afford such a vehcile.. So a lot of vehicles are decaying rapidly and thus folks are starting to scrap their Sleipnirs and try to buy new ones..
This inturn has put a high demand on vehicles... an with the same number of drops of BP's.. Demand is now outstripping supply and thus because there's a lack of supply, prices are starting to go through the roof... but it can only go so far..
eventually I'm guessing that in a future update, a newer set of flying vehicles will be introduced, and folks will be what will happen is that folks will buy the newer vehicles (and any subsequent weapons) , and folks won't want sleipnirs anymore.. so demand will go down. ad the prices will begin to fall..
It's just a case of usage, and demand that is driving the market.. no cartel, no short supply, end of story
Benjamin Ben Coyote a.k.a. "The Blind Sniper"