So what do I use my 51000 Old Castle Ruins exchange currency on?

The discounted Codex of Learnings for 4k each appear to technically be the most skill-exp for your bux.
But the expensive Intermediate Codex of Learning is still gives four times as much skill-exp as a Codex of Learning and it's just two times more expensive than the non-discounted, regular price for a regular Codex of Learning.
But for 51000 currency I can only afford two Intermediate Codex of Learnings.

But if I spend all my currency on two Intermediate Codexes, then I can't afford the Full Alteration Stones.
Have these Full Alteration Stones been available anywhere else? I should totally grab these shouldn't I? I am assuming they're pretty good for optimizing high end weapons.

So what should I do?
Two Full Alteration stones for 20000?
Then one Intermediate Codex for another 25000?
But now I'm already up to 45000, meaning that I can't afford both the discounted codexes that go for 4k each. But we can still buy one, landing on 49000.

That's two Full Alteration Stones. One Intermediate Codex and one regular Codex, for a total of 125 skill-exp. Do I just buy a discounted adventurer bone after that to add up to 51000 or what's best?
And am I overvaluing the Full Alteration Stones? I still think I should get them.

>all that shit just to move up 3 places when he could have just used his mega mushroom to move up more while collecting more items