for me i used the t1.micro but once this resources was too weak for the pipeline i had to change it to the next cheapest thing, m3.medium, you can use the link to compare prices (its already selected on the relevant instances just press compare)
the thing is you have to use HVM instance and not all instance families are HVM
you can see this table of Amazon AMI Instance Type Matrix:
https://aws.amazon.com/amazon-linux-ami/instance-type-matrix/
Instance Family:
1. HVM EBS-Backed 64-bit
2. HVM Instance Store 64-bit
3. PV EBS-Backed 64-bit
4. PV Instance Store 64-bit
and for cost you can look here:
http://www.ec2instances.info/?selected=t1.micro,m3.medium,c3.large,r3.large