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I disagree! Mathematics can be cool; and it's an indespensable tool for the good game designer. I've seen a lot of games with "cool" concepts that ended being unplayable because their designers did not have a sufficient grasp of the mechanics. It's a very common problem in the industry. I think realia are a cool concept. To analyse them, however, I think you should break up the two situations when they are "bought" with covenant build points, and when they are build in game by the PCs. I shall start with the latter.
Realia assembled "in play"
From what I've seen in my games, in a number of Arts it's not exceedingly difficult to get, over a few decades, a collection of about 1000 specimens. That's a level 11 collection (more precisely, 1024 specimens are level 11). Herbam or Terram come immediately to mind. The cool part is that the magus need not spend seasonal effort in doing so, though the player needs to get creative. He can have grogs roaming mythic europe and beyond collecting samples. He can collect them while on an adventure (they make really cool "treasure"). He can buy them from merchants. Etc. By the time a specialist magus is old enough that he's exhausted most good tractati in the field, he has the equivalent of 11 tractatus level 11, or 20 level 10, or 36 level 9, to keep studying without resorting to vis. Nothing truly extraordinary, but a nice little bonus with a lot of colour. Going up to level 14 or so (8192 specimens) starts to get definitely harder, but it's still feasible. At this level, you probably have all the study material you want for the rest of your life. A level 14 collection means 88 seasons of study at a (fair) quality 11, enough to push your ability in the Art into the low 40s on its own.
Realia bought with build points
The real problem with realia bought with build points is that, at low levels, they aren't really worth the cost, while at high levels they are really too powerful. Let's look at a level 6 collection, that costs 21 build points. I'd rather get two good tractatus, Q10 and Q11, that I can probably barter with other covenants for more texts of similar quality, than a level 6 collection from which I can study for 6 seasons with a paltry Q6 (or 24 seasons at Q3!). But at really high levels, collections are really worth it for the specialist -- this also mean that you can get a lot of profit by allowing other specialists to study from them. Consider a level 20 collection. That's over half a million specimens, or about the same number of books in the library of Alexandria. Legendary, but not outright impossible. It costs a whopping 210 points. However, it means 18*4=72 seasons of study at Q18 -- enough to bring a student to the low 50s on its own. It's a "supersumma" that gives a student all he'll ever need in one Art, and that up to 4 students can study simultaneously (or 8 with just a -1 penalty). In many ways, it's too powerful, though it would certainly be a central feature of the saga and make for a lot of fine stories.
Summarizing, I think realia are cool, and work well mechanically if assembled "in play". Bought with build points, they have the problem that at low levels they are not worth it, and at high levels they are potentially game-breaking and certainly saga-defining; a level range 10-14 seems a reasonable one (but note how the level 14 collection, which costs less than twice the level 10 collection, it's worth far more than twice as much).