Compare in-game currency to money.
Premium currencies exist to obscure price. Converting back is straightforward once you include any bonus coins in the denominator: pack price divided by coins actually received gives a real unit cost, and multiplying by an item's coin price gives what it costs in money. Two design patterns show up immediately in these numbers. Pack sizes rarely divide evenly into item prices, so a leftover balance is engineered rather than accidental, and the cost per thousand usually falls as pack size rises, which is what makes the largest pack look like the rational choice.
Real cost of in-game currency
Cost per coin = pack price / (coins x (1 + bonus)); item real cost = item coin price x cost per coin
Because a leftover balance encourages the next purchase. Pack and item prices are deliberately non-divisible, so clearing a wishlist almost always requires buying more currency than the items cost.
Per coin, usually yes. Whether that is a saving depends on spending the surplus on something you actually wanted — otherwise the cheaper unit rate is bought with money spent on coins you never use.
Substantially, and they must go in the denominator. A pack advertising 10% bonus lowers the true cost per coin by about 9%, which is enough to change which pack is the best value.