medea, the page where (I assume) you retrieved the picture lists that "The Twentieth Anniversary Mac was a limited edition, and sold for nearly $10,000. Its price was cut to as low as $1,999 and it was discontinued in March 1998."
http://www.apple-history.com/quickgallery.html?where=aniversary.html
As for a Twentififth Aniv, 30, 35, etc., I strongly doubt it. The machine had an abundance of extra and leading tech (for 1997) features, which really makes this more Powerful than the PowerMac.
The idea of the iMac G4 being the 25th Aniv. Mac sounds like nonsense, since it doesn't cost $10000, doesn't have a sub, and doesn't have FM Radio built in, except for a radio tuner in iTunes.