IMHO, Eduardo is correct. You want ERP for free? Really?
I'm personally glad that nop leaves inventory costing (LIFO/FIFO) up to us to support because that retains its breadth of applicability that is so appreciated. Hire/contract a competent (read: full-stack) Developer to customize your cost cycles and reporting. Or buy an ERP, related infrastructure and then hire a team of specialized Developers to support it.
Please don't ever morph nop into a convoluted massive system that only supports 10% of us and still requires professional customization but on a much more complex infrastructure (read: $$$).