Cost is also not the only consideration in the decision to develop custom software, as the requirements for a custom software project often includes the purchaser owning the source code, to secure the possibility of future improvement or modifications to the installed system to handle changing requirements. However, modern COTS often has application programming interfaces (APIs) for extensibility - or occasionally, as in the case of Salesforce.com, a domain-specific language (DSL) - meaning that COTS packages can sometimes accommodate quite a wide variety of customisations without the need to access source code of the core COTS system.
Custom software is often several times the cost of the other two options, and will normally include an ongoing maintenance cost. This will often make custom software infeasible for smaller businesses. These higher costs can be insignificant in larger businesses where small efficiency increases can relate to large labour cost savings or where custom software offers a large efficiency boost.
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