Replacing $4.99 plants into seed packets

When they released Penny's Pursuit last Febuary, plants such as Hot Date (from $4.99 to 200 seed packets), dandelion (from $4.99 to 150 seed packets), and other plants (from $4.99 to 250 seed packets) they replace some plants to unlock by using seed packets instead of real money. Another problem is that plants like Power Lily remains at $4.99 instead of replacing it into seed packets, why do plants cost real cash?