Hi:

Basic steps to costing a project in hours:

1.)  Create a context diagram to determine the scope (extent) of the system.   Be sure to walkthrough the diagram to verify it.

2.)  Properly "chunk" the system into equal-size pieces of a size small enough that you feel comfortable estimating the  hours required for that piece. 

3.)  Estimate the hours to complete that  single piece of software.

4.)  Multiply the hours estimate for the single chunk by the total number of piecess .

5.)  Adjust upwards the above calculated  hours by a factor for various inefficiencies

Tony