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
Rabble Rousing Rabble Rousing has one of the easiest hideaway conditions to fulfil, and facilitates reaching the 10-creature threshold all on its own
And once activated, it triggers a powerful effect
Mirrordinsanity is a collection of five Artifact Land cards featuring the psychedelic illustrations of artist DXTR, while Showcase: Midnight Hunt recreates ten of the recent set's basic Lands in its black and white showcase art style
Mana Value 9: Grozoth For those players who desperately need to tutor up a seven- or eight-mana spell, Transmute unfortunately doesn't offer a solution