insert text in textbox on sum output

Question

Guy's,

I'm trying to insert a value into a textbox if the value in my sumTextbox is above £250.

Example I have a two textboxes one just called textbox1 and one called sumtextbox, when I run a function (button click) I would like to check the value in sumtextbox and if it's above £250 it inserts a value in textbox1 of "YES"

Is that possible with a button click?