Multiple versions of the same application - what to charge?

I recently delivered an application to a new client, who is very happy
with the work. The client has asked me to develop several country
specific versions of this application. These are not language
versions, everything will still be in English, the only difference is
a couple of simple and very minor cosmetic tweaks. I billed around
£6,000 for the original application. The work for the new versions is
minimal.

I have never got in to licensing or any specific "deals" before, I
have always just billed for my time. I often hear of developers
"licensing" applications for astronomical fees and thought maybe I am
missing a trick.

I can't put my finger on it, but I feel I am underselling myself if I
just charge my hourly rate for this work; what was a 10 user app could
now have dozens of users all over the world, should they pay peanuts
for my expertise?

How would you price something like this?

Paul