A lot of balance and care goes into choosing the cost of a card, more than I can express here, but looking for similar effects on existing Magic cards can give you an idea of a fair costfor example, a ramp spell that puts two lands into play ought to cost 4 mana, per Explosive Vegetation
Graveblade Marauder and Ukud Cobra are very good with fight effects
If you can find one or two at a garage sale, or a stash in someones attic, it can amount to major profit
It is common for a description to sound like a high-rarity version, but upon looking at the photo, it is actually a different specification
Stage 2 Pokemon are traditionally among the most energy-intensive cards in Pokemon Pocket, but Shining Revelry is scaling that back by introducing many Stage 2 Pokemon that can attack within two turns