Haven't been using Axure for long but I think I may be able to answer your question.
So you have a Text Field into which the user enters a number, and you want it to change the state of a dynamic panel if the number is larger than 125001.
Select the Text Field, and go to Interactions. Add an "OnTextChange" interaction and choose the "Set Panel State" as the action. Select the DP you want to touch and the state it should go to.
In Axure RP 9, which I have been using, there is a small "IF" button on the interaction which I can click to enable conditional logic. Entering conditions like the ones in this screenshot seem to work.
Hope this helps!