ERP5 is an open source ERP. The source code of ERP5 is freely available on Gitlab and instructions on how to get started with installing a virtual machine to evaulate ERP5 can be found here.
Traditionally ERP systems are often preconfigured, for example tailoring to a specific industry sector (like ecommerce or automotive). ERP5 is different in that it is a very generic framework with a core that can then be extended with packages (called business templates or bt5) that provide additional functionality like accounting or trade. Packages itself cover a basic set of features (accounting schemes, sales order management) and are usually then customized to match the specific needs of a company (specific accounts, specific way of managing orders).
There are two ways of customizing ERP5: Learn to develop ERP5 yourself ( ERP5 Developer Documentation) or ask Nexedi, the maintainer of ERP5, to implement a prototype or production-ready application according to your specifications.
As ERP5 is a complex framework, the learning curve is quite steep and becoming a proficient ERP5 developer may take up to a year. Projects implemented by Nexedi are therefore either too complex to be done elsewhere or too urgent to invest the time to learn ERP5 from the ground up (see the success case section on the Nexedi website for sample implementations).
Projects can range from a couple of months to multiple years depending on the system requirements. Usually a project budget does not start below €50.000 and can grow to several million depending on overall scope.