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
My request for the design was that I wanted a dragon that did something dragons don't normally do
While splashing removal spells can definitely be correct (either because its convenient or sorely needed), splashing game-winning cards tends to be right more often
"The most expensive card I knew about was Post Malone buying [a unique] artist proof Black Lotus for $800,000
A staple in multiple formats, Cryptic Command is one of the strongest blue spells around
Rhystic Study Those aware of Rhystic Studys Commander dominance may be surprised to hear that this card was originally printed as a Common