Cards worth sending in on $4.50 special??

Just wondering what you guys think a card must be worth to send it in for the 4.50 special? i.e. is a '64 common worth it if it might be an 8? I'm just talking about a regular common, not a low pop common or anything. Thoughts, advice, etc...
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<< <i> i.e. is a '64 common worth it if it might be an 8? >>
Nice example. I just sent off my 4th and final sub of the month that included three 64 Topps commons that I think have a shot at an 8.
If it's for your own collection, then you only need to ask yourself "Is spending a $4.50 "slabbing fee" on this card I already own worth it, or would I be happier keeping it raw and spending the $4.50 on something else?"
WTB: 2001 Leaf Rookies & Stars Longevity: Ryan Jensen #/25