How can I calculate database design storage costs?

Question

I often have a couple different schema's in mind when starting project. After making rough guesses I realize that some are less optimized for growth or storage space than others. Obviously, the size of the column value is the main thing. But table metadata, indexes, and row headers all play a part as well.

In addition, RDBMS use a completely different approach to data storage than object or key-value databases.

What are some good resources for trying to figure out the cost (or room needed) for database storage?

Note, my question has little to do with choosing the database, but rather knowing how to properly make use of each database's design for the most efficiently. Databases like PostgreSQL, MySQL, CouchDB, all have different target use cases and multiple ways to solve the same problem. So knowing the storage cost of each solution will help add to the choice of the best solution for the schema.