Have some sort of variable first, either an instance variable (for the player?) or a global variable if you want more control/less possible bugs. Create a text object, and put it on a layer above the other layers (you know, the layers you have for objects, the player, backgrounds, etc), and set the parallax and scale rate for this new layer to 0.
On any event that you want to make changes to the player's available money, do your general stuff (subtracting/adding from the variable) but include an event for the text object created, and select the action to change said text's content. Set it the variable. Just type in your variable name (if global) or if using an Instance variable:
Player is the name of the object that has the instance variable, and Money is the name of said instance variable, which has to be a number type and not boolean or string.
Another way to go about setting the text is instead including an "Every tick" event at the very top (after any On start of layout events), and in said event, change the text object to the variable.