Is what you are saying that you want to change the way the button looks dependent on whether the price is $0 or not?
If so you are in the right place with tpl_product_info_display. Personally, I would build two versions of the $button. Your if statement will look something like:
Code:
<?php
$ddd = zen_get_products_display_price((int)$_GET['products_id']);//gets the display price
$ddd = substr($ddd,1);//strips off the $ sign
$ddd = str_replace(",","",$ddd);//removes the commas that are used to separate thousands for weird American reasons
if ($ddd = 0){
//build one button
}else{
//build the other button
}?>
Exactly where you put this depends on exactly what you are trying to do and what options you are using but it looks like you want to wrap it around the line (as ' build the other button'):
Code:
$the_button = '<input type="hidden" name="cart_quantity" value="1" />' . zen_draw_hidden_field('products_id', (int)$_GET['products_id']) . zen_image_submit(BUTTON_IMAGE_IN_CART, BUTTON_IN_CART_ALT);
Then repeat it but with a different image specified etc. for the 'quote' button. ie in place of 'build one button'
As I write this I realise it is not the clearest explanation I have ever created
So get back in touch if I have confused you.