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
Rosie Cotton of South Lane There are a lot of ways to go infinite with Rosie Cotton of South Lane
Curiosity (Ext, S, Block) The best card drawing creature enchantment in all of Magic , Curiosity seemed to consistently enchant various Merfolk
His non-Enchanted variant is a Legendary Rare, which is still reasonably pricey
It's also a cool card for any fans of the character, depicting her in shining armor that makes for a stark contrast to her more common blue ball gown or the simple dress she wears before the Fairy Godmother visits
The FIN starter deck features characters like Xande, Seymour Beta (likely Seymour Flux), and Artemisa (which is probably Ultimecia in English)