Based on the latest market prices tracked on TCG Stacked, Exploration is worth about $32.37 USD for a regular copy and $37.93 USD for a foil copy
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If at all possible, you should make Letter Bomb look the same
Other cards stay at pretty much the same price, whether they're graded or not
I think in this case, the previous cards with Magmortar firing were deemed acceptable because they could be passed off a fantastical Pokemon attack, while perhaps this time TPCi thought that the smoke was too suggestive of a fired hand gun, which is a type of weapon that is more accessible to children than Pokemon fire or a mortar/bazooka, and changed the pose for that reason