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.
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.
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.