Yeah it's definitely not a great way to stack but if you like the design and enjoy collecting them it can be a fun thing to do and it does look cool when you have a full set. Have you seen the 50 peso gold coins? They're massive! 1.2 troy oz of pure gold.
I heard the late 1990's are the hardest to locate and the most expensive as well due to mintage numbers but I truly don't know the facts at all. Did you get most of them off of the bay or from coin shows? just wondering.
I've been at it for awhile. I have always checked the coins shops you never know what is going to show up. A lot of the 90's date came from a couple of different dealers that are in my local shows. I have bought some from ebay but not that many.