@Yasser530 While technically the minimal capacity reservation is 100GB/day if you look at the amount of the discount you get FOR THE AZURE SENTINEL billing, it makes sense to start using it when you get to 50GB/day since the cost would be the same since Azure Sentinel gives at least a 50% discount when using a capacity reservation plan. This is different than the Log Analytics billing as that only gives at least a 15% discount for going to a capacity reservation plan. The Azure Pricing Calculator, https://azure.microsoft.com/en-us/pricing/calculator/, is very intelligent in this regard and can tell you when to use which capacity reservation plans. Keep in mind that you do NOT need to have the same capacity reservation plan for both Azure Sentinel and Log Analytics.
The total price for Azure Sentinel is the Azure Sentinel Ingestion + Log Analytics Ingestion + Data Retention (first 90 days free). There is also a charge if you use Playbooks (charged at the Logic App rates).
As far as educational discounts go, you would need to talk to your MS Licensing rep to get a definite answer to that.