What is the cheapest way of remote monitoring of field devices or instruments,I want something that will enable me to view these devices from home, they all are using 40-20mA as an output and they have Ethernet Port
If all devices are connected to Scada then you can connect Scada station through a remote PC. You have to configure only.
Other options are a little bit complex.
Like OPCUA and other options
Use an industrial router with gsm sim or any device that can connect to the internet. Use modbus protocol and pull the data from the devices from your computer. You can use a scada application from your computer.