The Not-So-Splash Page

So I got hired to design and develop a splash page for a client. After a short conversation it was clear he wanted a one-page website to hold him up until he launches a more proper site.

I still charged him for a splash page, when a one-pager is clearly more work and takes longer to create. I told myself it's a good start to a client-designer relationship.

At what point do you (the designer) become a victim of exploitation, or put yourself in a financial corner undercharging?

How do you guys go about a situation like this? Do you give the client the benefit of the doubt, or do you reflect the change of scope in the invoice/estimate?

PS: Let's all pretend that you can't see any cables on my messy desk in the reflection.