I'm not interested myself (I have a 100% 
Suppi/Ruby Moon/Eriol gallery), but I do know 
that since the character appears much less often 
than Kero-chan, he's always been in somewhat more 
demand than other magical characters.  On the 
other hand, since the character design is fairly 
simple, Suppis will never bring as much as Yues 
(or Spinel Suns, for that matter).  But I'd guess 
between $50 and $100 depending on size of image 
and condition.