A fair price for a Japanese Pokmon booster box is typically between $45 and $65, depending on the sets age and popularity
The main bottleneck you will face is hitting 2 lands or running out of mana
Discarding an island is just much more difficult than discarding a blue card, especially with greedy mana bases, but its still doable
Like most Pokmon cards, the value of Manectric depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Platinum)
The Silver-Bordered Project is supposed to provide clearer guidance for the Magic the Gathering community when making judgement calls about whether or not to include cards in casual games
Mega Greninja is in a different conversation entirely