I have no doubt that the data says something different from "if it's more efficient to work for minimum wage, people will just work a job instead of grind". And now, putting on my game player's hat, as someone who's played diablo 2 for the longest time, been with Alteil since set 5, etc...here's why:
It's the idea of obtaining in-game items through in-game means. It's the idea of not breaking the "magic circle"--that barrier that exists between the real world (where you earn money for everything), and the game (which has another, in-game way to earn those same cards). In essence, it's the idea that who wins matches shouldn't be determined by who has a better real world job (and as a budding data scientist, I very well may be out-earning most if not all of the people on this board in a few years). This certainly gives me enough leeway to just load up a bunch of gran, get all the cards, and go cruising (I'm on AA's account doing that sort of doing that right now, but I can do similar stuff on my own account if I felt like it). But that would defeat the purpose of taking the actual journey to obtain those cards through in-game means, and make my wins feel illegitimate.
Furthermore, still wearing my player's hat, this time my Alteil player's hat, there just isn't enough reason to spend $5-$10. $10 might get you one playset of EX. However, what about the regular sets? $5 would get me 500 (600?) gran. I buy 2 gold packs with that. What are the chances that I get a playset of Ben Maclachlan? Tiny. In essence, right now, we have "wait for hyper lotto for your 1 and 2*s and occasional 1-of useful 3-5*", "save up gran for your sphere's EX" (and if you're playing two spheres? One of my only instances of spending was for Belltia because I got EX6 Falkow already because EXwiz was so good at the time). I heard that when you released the prebuilts for 500 gran went down, and you're loathe to do it again.
Now, putting on my data scientist hat, I have a hypothesis as to why: I think your revenues may actually work like a movie theater's. When a new movie comes out, everyone flocks to see it. Then fewer and fewer people, and after a few weeks, everyone who wants to see it has seen it (still gotta see The Hobbit >.>). Same deal with spending. If someone intends to spend a good bit of cash to get the cards they need from a set, they're going to buy that set on release, or their first immediate opportunity following the set's release, until either they say "enough", or they get the cards they need. If you compare prebuilt sales to set releases, well, of course they'll decrease the overall spending, because they allow access to great cards for the price (cudgel SS? Chiruru SS? EX Riza? EX Zuga? Rasam and EXWiz and Vosrot? Dub Rido?).
That said, I think the prebuilts revealed something--that it isn't the omfg amazing rares that should encourage the low spenders to spend, but that people just want a half-decent file to play. They don't need a file chock full of 5*s just to put up a good fight in Folrart. They can scrounge up point cards for their Bringers/Allinds, and hope to get lucky on the level ups (speaking of those, why not offer an option to take 45 point cards instead of a set 5* at level 100+?).
At the moment, I think that there really is no place for low spending. It's either shell out a crapton for entire sets/ex boxes, or play free, because anything in between is a complete crapshoot. So long as it stays a crapshoot, there's no real reason to spend a little here, a little there.
So yeah, maybe you should set a respectable level requirement for auction house (50 seems decent--it's after the 30 at which you get your last 3*) just so people wouldn't troll Crest with alts, but that's just a risk you'll have to accept--that at some point, someone's going to be so dedicated to the idea of the magic circle that they'll go in Crest and troll the little newbies. IMO the answer to that is better grinder events. Like the stuff that got us Afeemina/Resource, the 60 PCs, etc...
Basically, I think if you set up the business model correctly, there will always be a place for those that just have to have every last card in existence, or make those chock-full-of-5*s files, and they'll still pay handsomely. But there needs to be a lot more "if you just pay $5-$10, you can have a very respectable file, and if you combine it with good PC usage and some dedication to the game (grinding), you may very well end up a top player". Because that's what this game is missing IMO.