There are a number of factors that determine the overall cost of implementation. If you work in industry and are looking for an estimate as well as resources for implementing Oracle ERP, I would start with a Request for Proposal process (or RFQ if you already know the parameters and scope of the project). Going through this process will help you understand the scope and complexity of what you are about to undertake, as well as give you a better idea of costs and where those costs are incurred (software, hardware, internal and external labor, etc.). There are a number of consulting firms out there - both big and small - who specialize in Oracle and other ERP implementations. One other option for you would be to look at a hosted installation, where your implementation partner would not only help you implement your ERP, but host and maintain it as well.
Regards,
Michael Siebert