Some SCADA packages are SQL based (wonderware, iconics) even historians (inSQL, SimaticIT) so you can hit the DB directly with JDBC drivers. That will cut out all PCO and OPC Servers inbetween. Do you have an MII license? SAP licenses everything now like small, medium, large. That's usually the biggest cost. Everything else is peanuts. For actual value, speak wit a SAP rep. No one will give you quotes here since most of the time the cost is negotiated and depends on a lot of factors.
Kepware provides OPC 3.0 connectivity with about $1000 USD per package. your OPC server needs to be compliant with 1.2a version of the protocol, or better than that. RSLINX (from Rockwell) is usually NOT compliant and not certified - and you'll have issues in transferring WORD tags and FLOATS.
If you cheap out a good software link, what you get is an unstable solution, which will eat 10x the cost of a good software in maintenance.