The cost will be up to you.
If you want the AWS tool to poll the Modbus, then trying to hold open a socket all day, every day, will cost about 2-3 MB/month. Add to this your traffic, but be aware that VZ (all carriers) charge you for all the TCP and IP overhead (headers, ACKs, retries). So if you look at the raw Modbus & calculate 1MB/month, the overhead could make this 6 to 10MB/month. They also 'round-up' every hour, so more overhead is added each time you talk (or per hour if you hold the connection open).
If your device somehow drives the connection to only link when needed, then a TCP/IP socket can consume say 1000 bytes per session. SSL/TLS would be in the 3-5K range.