First, as an EM collector, let me first say that $500 for your Fireball was an excellent purchase! Congratulations on snagging a great game! Once you get it cleaned up and working, that game immediately becomes worth $1,500-$2,000. Very nice!
Second, while Fireball is an iconic 1970's pinball machine, it is in no way close to being the MM of the EM World.
In the today's EM Market, Gottlieb Mermaid & Gottlieb T.K.O. are closer to being the MM of the EM world than Fireball in terms of value. Both of these games are valued in the 5 digits. The last Mermaid sold for over $25,000 I believe. The last T.K.O went for $10,000 I believe.
And many Gottlieb Wedgehead Machines can easily surpass the $2,000+ level. Just reference the Gottlieb Atlantis that sold recently on eBay for $3,000.
EM games can be found for a very cheap price. But the desired games that are restored consistently crack the $2,000 level.
Marcus