How the calculator works out the price
Every pack on the UK App Store has a fixed price, so the calculator tries every mix of packs and keeps the cheapest one that gives you at least the coins you need. For 1,000 coins, enough for one Galaxy, that's four 210-coin packs, two 70-coin packs and one 20-coin pack: exactly 1,000 coins for £12.85.
Buying the same 1,000 coins in 40-coin packs would cost £14.75, which is why the calculator shows the cheapest mix rather than a single rate.
For very large amounts, it buys most of the coins in the best-value pack (210 coins for £2.69) and finds the cheapest way to cover the rest.