Custom Merge Tag Code Issue

I want to show balance info by a custom merge tag. Let me show my code first:

global $current_user;
get_currentuserinfo();
$id = $current_user->ID;
$balance = balance($id);

I need to display $balance value into forms (actually in a HTML block) via merge tag.

How the merge tag code should be?

NOTE: I know how to autopopulate fields by a function but as I'll use html block, I need merge tag.