I found a solution for this anyway. I used the this.point.shapeArgs.r
to get the radius of each bubble in formatter function.
here is the function:
formatter:function(){
return "<div class='labeltext'><span style='position :relative;top:-"+this.point.shapeArgs.r+"px;display:block'>Group #</span><span style='position:relative;top:-10px;text-align:center;'>"+this.y+"$</span></div>";
}
full updated example is here
Update To solve the "data labels being cropped" issue, take a look at this updated version