simple input on WP from dashboard

Hi,

I’m looking for a way (a plug-in, maybe?) to give a client a place to enter one number in the WP-admin section somewhere that sets a variable in the db that I can call on his WP site.

For example: the client enters 500 into a box and clicks SAVE. On the site, I display “$500 raised to date!” So the [500] is being called from the client’s input.

This seems unbelievably simple, though I can’t seem to find a plug-in or method that simple enough for the task. Any ideas?

Thanks!