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Oh god. I played around with this before. It's not easy to find a good balance between reasonably cost and wow factor.
Tthere are a few ways to make it easier:
Add some colored mana to the cost so it always costs something
Make the stats low enough that it isn't so bad if it's cast on the cheap (think Frogmite ).
Make less of the card's power come from high stats so it has less of a dominating presence on the battlefield. Give it a cool ability instead.

Fair enough.
Alright so if this is a colorless 5/5 then I wouldn't want it hitting the field before turn 5 except in decks dedicated to getting it out. Let's say your average deck has three/four lands and three nonlands out by turn 5, then this guy would get a cost reduction of 6 or 7 mana by then. We'd still want him to be someting you'd tap out for at that point so that would indicate a cost of ~.
Then of course you'd have to look out for easy combos (especially in the standard environment) and up the cost as necesary. I would probably ballpark it at, say .