Why does IT cost so much?

I recently visited with a potential customer for a web application I maintain. They were enthusiastic with just one little caveat, could we make a functional change to the application?

I said, "Sure, let me work up a proposal for you and we can go from there."

Once upon a time, I had a tendency to under-price project work. Now, I look at all of the costs involved in making changes and base the price off of that. When the potential new customer saw the estimate, there was a moment of sticker shock, "So much? I just wanted this one little thing."

And then it hit me, the perfect analogy for why modifying a system with over 100 client organisations can be a bit pricey. Here is what I told them:

Imagine a house that lots of people visit every day. What you are asking for here is, essentially, a new room to be added to this house. Now, if this new room would be useful to all of my visitors, I might just do it for free, knowing that this addition will raise the value of the house as a whole. The room you are asking for here will be behind a locked door, but it takes exactly the same amount of effort to create as the open room.
This new room must be solid, and be soundly designed. Because this room will be attached to the house, it has to be safe and it cannot be unstable, as that would make the whole house unstable. I need to run wires and plumbing into this new room, and that means changing the plumbing and wiring in other parts of the house as well.
This new room, once it has been built, will need to be maintained. I have to consider how new things in the house will affect this new room. When the rest of the house gets a coat of paint, this new room has to get a new coat of paint. If the house gets termites, I have to spend time and money cleaning up this new room as well.
So you see, it's not just the room itself, it's the room and how it connects to the house and ongoing maintenance.

The client 'got it'.