You can use PROC NETFLOW to solve min-cost network flow problems. You can also use PROC OPTLP or PROC OPTMODEL - if you formulate it as a standard linear program.
Also, we are currently developing a new product that will focus on graph and network flow algorithms - called PROC OPTGRAPH. It will include a new and improved solver for min-cost network flow. It will probably be released in 2009 (maybe 2010). If you are allowed to share your data, I would appreciate getting a copy. I can use it to fine-tune PROC OPTGRAPH since it is still in development.
Let me know if you have any issues using NETFLOW or OPTMODEL.
Thanks,
Matt -- matthew.galati@sas.com