It's soooo easy to me.
The important thing is you must create a global variable for score, besides 2 options on menu screen, you may create event compare to global variable, if the current score is less than 49, it will disable click, otherwise if is greater or equal than 50, it will enable click to purchase weapon, it does same logic to other weapons and add action to substract 50 to global score after of purchasing a weapon, also you make another event to attach new weapon, it?s just like to change frame, add image point or other method.