A computer program has so many variables, that it's difficult to estimate the time required to estimate the time required to give you exactly what you want.   Now I've heard - "I know exactly what I want!"  Okay, do you know what every piece of data, every field on the screen and every piece of data on a report should look like and act like?   If you really think about it, you don't.

Let's try a little example.   Let's define a simple program to track phone calls.   Let's keep it simple and say only one person is going to enter information into this system.   Now, we know we need the obvious: time and date, who called, a topic plus maybe a notes field for details.   Simple, but do you need first name and last separately - are you going to want to look up by last name or first?  Do you want to type the names in all the time or do you want to be able to look them up?  Do you want it to type in the date and time or do you want a button to automatically enter it for you? 

Do you want to track who they were calling for - or what they were calling about from a list so you can keep reports on it later?   Do you want to track the resolution of the phone call?   Whether they need a call back or not? 

And so it goes down to the color of the screen and buttons.

Some people say my home builder can give me a price why can't a programmer.  A home builder has plans that you've either purchased - or had made.  This most likely has cost you thousands of dollars.  Programmers can do the very same thing but you have to go through a detailed analysis document before the programmer can start and just like building a home, this costs thousands of dollars.  Most customers do not want to pay for this.  The alternative is to build in small phases to make sure the program is built how you want it without huge mistakes.

For those of you who have had your own home built then you know about the things that are missed. Enough electrical outlets and in the right spots, enough lighting, enough cupboard space, enough room here, doorways, ventilation, etc.

In simple terms, programs have two flavors, one your computer understands and runs, this is typically called the executable.   The other is the source code which is in a computer language that a programmer can understand and work with. 

Without the source code your program cannot be modified, or updated by anyone other than the original programmer who has the source.   If you are paying for programming by the hour then you are paying for the source code and not just the executable.   Don't hire a programmer who is not willing to give you the source code and make sure this is in the contract.

There can be circumstances where you might not get the source code - you are paying a fixed price for the system with an understanding that the programmer is going to resell that program or it is a commercially licensed system that is sold publicly, eg: Quickbooks.   Anything else and you are being held hostage, you'll always have to go to them to get modifications done, and they can and typically do start raising their rates.