Many software are developed from Open Source. There is a crucial need to estimate the efforts spent in a project development based on Open Source. These estimates give room for comparison between the development model based on Open Source and others and lead to a decision making concerning a project, if it is judged worthy to be continued, discarded or combined with traditional approaches. In the present paper, a case study is made using approximation of functional size to the software based on Open Source in order to show that the development of applications based on Open Source has many advantages in terms of efforts. For this purpose, the present work sets out to adapt an Open Source software called TRIADE and to evaluate its functional size matching. The results show that this adaptation can decrease the functional size of software up to 90% compared to a development from scratch. The present study might constitute a research frame on the development and/or validation of a model of cost estimate of application development based on Open Source.