Do they have to disclose how much they paid?
Sometimes you can find a reference to it in the buyer's the next quarterly (10-Q) or annual (10-K) filing, but since again this is small, Apple will likely only mention it in passing and any sum paid will either be missing or lumped with a bunch of items to make it undiscernable. No upside in disclosing
And then there's the issue of value vs dollars. Even if we knew how much they paid in dollars, we wouldn't know how much it was worth without some metric for what they bought.
The vanilla public-to-public case for stable industries like widget manufacturers is one company buys another for their future earnings or cashflows, hence ratios like P/E (Price per share divided by earnings per share) or Enterprise Value/EBITDA ((Market Cap+Debt+SimilarStuff)/EBITDA, with the usual caveat that EBITDA is not a perfect proxy for cash flow but it's somewhat more observable)
In this case they likely bought IP and possibly some talent, so valuation there is a bit different. I don't have much experience with acquihires but I guess it just comes down to how much these specific folks required to take the deal rather than walk away...
Since the startup is quite young I would also agree Apple could be interested in their talent.
> The report says that Datakalab’s two founders did not join Apple, but multiple other employees did make the jump.
Perhaps they're interested in IP, too. In any case, I don't quite know how to read Apple's strategy of picking small startups (PA Semi and Anobit being, in retrospect, surprisingly small for their impact).