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
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The front side depicts Peter getting his iconic spider bite, with the backside in his superhero persona
On top of that, youre rewarded with card draw whenever landfall triggers, making Aesi, Tyrant of Gyre Strait a perfect addition for the set
Deathgrip + Lifeforce Unexpected counter resources that break the color pie and scream Magic boomer to the table
Open an account, and you'll start with hundreds of cards just for signing upenough to play around with multiple strategies and upgrade your decks before you ever have to open a card pack