Interpolation

Tier A: 100 items £3
Tier B: 200 items £1

Price of Tier A: Pa
Price of Tier B: Pb
Price of the item: P

Quantity of Tier A: Qa
Quantity of Tier B: Qb
Quantity of the order: Q

P = (Qa * Pa + (Q ‐ Qa) * (| Pb * Qb ‐ Pa * Qa | / | Qb ‐ Qa |)) / Q