- PostGIS/PostgreSQL - $0
- ArcSDE - $18,000+
You also have to keep in mind that ArcSDE is only a geospatial layer that must sit on an enterprise database system like SQL Server or Oracle. This means the cost can be significantly more than this.
Now this cost savings can easily get nickle and dimed away if you spend alot of focusing on trying to get the OS GIS software to do things that it won't easily do. The key consideration here is PLANNING. Make sure you budget for enough up-front planning and the design, development and implementation will certainly be much more efficient. There are many techniques used by GIS architects and designers to make sure that the software you choose, either open source or commercial, will do what the project requires. I always recommend getting the advice of a consultant before taking on a project with technologies with which you are unfamiliar.