Tjyylli

A spellshard is a flavorful replacement for a mundane book, and among other things, a wizard can use one as a spellbook. On this subject, the item description says:

An arcane caster can use a spellshard instead of a spellbook; the spellshard costs 1 gp per “page” in the shard, and otherwise functions as a mundane spellbook.

—Spellshard, Wayfinder's Guide to Eberron, pg. 115

It's not clear to me what the "1 gp per 'page'" means. The fact that this clause occurs in the middle of a sentence about using the spellshard as a spellbook seems to imply that it is specifically related to this use. Is 1 gp per page an additional cost to copy spells into the spellshard? How many "pages" is one spell? Does the spellshard hold a limited number of "pages", and does this limit the number of spells that can be copied into it?

In short, what practical differences exist between a mundane spellbook and a spellshard being used as a spellbook?

There are, in my mind, two possible ways to interpret this item, starting with what I think is most likely/intended.

It costs 1gp to add pages to the Spellshard

This isn't explicitly stated, but there's two parts of the item description that lead me to believe that this is the case. One is the section you quoted, and the other is this:

Thinking of a particular phrase or topic will draw you to the first section that addresses it, and a simple ritual allows you to add content to the shard.

—Spellshard, Wayfinder's Guide to Eberron, pg. 115

The way that I interpret the confluence of these two passages is that the ritual to add "content" to the spellshard costs 1gp in some kind of consumed reagents for each page that is required to be added to the Spellshard. Conversely, there's no reagents required if the "adding content" is simply filling out or altering a page that's already in the shard.

What those reagents are is unspecified, which means it comes down to your DM.

Alternatively...

Spellshards have a Finite capacity; the price is determined by their page count

This is more straightforward: A 300 page Spellshard costs 300gp. You can't add pages to it, but you can use a ritual to make alterations/additions to the contents of the Spellshard.

This seems like a more straight-forward reading of the item description, especially since the description doesn't explicitly say how to add or remove pages from the shard. I do believe the above is a reasonable interpretation of the abilities and features of this item, but I suspect this latter interpretation is closer to the Rules-As-Written.

Either Way...

The item says it can act like a mundane spellbook. That means for a Wizard:

• You may add spells to it (using the normal costs for copying spells)


• You may prepare spells from it


• You may cast Ritual Spells contained within it as Rituals, even if the spell isn't prepared when you do so


• ... Anything else that a normal Spellbook is able to do, minus any particular constraints of the shard itself.

In the case of your specific questions: it depends on which interpretation you go with, but at my table, it would cost 51gp (26gp if it's from your school) to add a spell to this spellbook, unless it had free, blank pages, in which case it would simply cost 50gp (or 25gp). This might seem strange, considering that you cannot physically use the ink to transcribe the spells into the shard, but bear in mind the 50gp/25gp cost isn't solely ink:

For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it. Once you have spent this time and money, you can prepare the spell just like your other spells.

—Spellbook, Player's handbook, pg. 114

So it's not just inks used to transcribe the spell; perhaps you could argue with your DM whether there should be a discount since you don't have to use ink? That's a decision they will have to make.

Standard spellbooks are vellum books with 100 pages each, initially blank. They cost 50gp each.

Standard spellshards are magical book-equivalents with varying numbers of effective pages, initially blank. They cost 1gp per page, and otherwise function as a standard spellbook (including costs to copy spells and so forth).

It's a bit more expensive per page, but it looks cooler, and it can potentially hold more pages than a standard book (or less) if that's important to you. It's probably lighter as well.