Is there a low-cost way to track requests from/to AWS resources?
It really depends exactly what your requirements are. Do you need to log the user logging in, or just the IP they're from?
If IP is sufficient I suggest that access logs are probably the cheapest and easiest method. They're definitely available for S3, should be available for software running on EC2. RDS i'm not too sure but a quick Google suggests database specific features may be able to provide something that will meet your needs.