a1.medium is an AWS EC2 instance with 1 vCPU, 2 GiB memory (AWS Graviton Processor). In US East (N. Virginia) it costs $0.0255 per hour on demand ($18.62 per month) for Linux; a 1-year reserved instance with no upfront costs $0.0161 per hour, 37% less. Spot capacity is about $0.0239 per hour. It is sold in 9 regions; the cheapest is Asia Pacific (Mumbai) at $0.0255 per hour.
Prices by region
USD per hour, Linux. Reserved prices are no upfront, standard. Cheapest on-demand first.