If your planning to buy for an 1 year Convertible Reserved Instances provide you with a significant discount (up to 66%) . On-Demand Instances provides upto 1year to 3 years.
It depends on instance type, instance size, operating system, tenancy model, region, and availability zone, as well as whether you use load balancing.
The most p3dn expensive EC2 instance is 24xlarge which costs $31.212 USD per hour when running a Linux OS and $35.628 per hour when running the Window OS .The instance comes with 96 vCPUs, 768 GiB of RAM and two 900 GB NVMe SSDs.
Yes EC2 has a high availability if your using Amazon EC2 instance . Amazon provides built-in capabilities to achieve high availability.
Amazon EC2 High Memory instances have up to 24 TiB of memory.