A serum typically costs several times more per millilitre than a cream from the same range. Some of that reflects real formulation cost and some reflects where the product sits in a routine.
Serums carry the actives
A moisturiser is mostly oils, emulsifiers, humectants and water, all inexpensive at the volumes cosmetics are made in.
A serum is built around specific active ingredients at meaningful concentrations, and those ingredients can be orders of magnitude more expensive than the base.
Which means the cost difference is real at the ingredient level, though it is usually smaller than the price difference at the counter.
Stability is harder to achieve
Many actives are unstable in water, sensitive to light, oxygen or pH, and prone to degrading before the product reaches the end of its shelf life.
Holding them stable requires encapsulation, buffering systems, specific packaging and more development work than a straightforward emulsion.
Airless pumps, opaque glass and single-dose formats all add unit cost and all appear far more often on serums than on creams.
Small volumes read as concentrated
Serums are sold in small bottles, which reduces the headline price while raising the price per millilitre substantially.
Small packaging also signals potency. A tiny bottle implies a product used sparingly, which supports the impression of concentration.
The format is genuinely appropriate for unstable actives, since a smaller bottle is used up before it degrades, but it is also commercially convenient.
Routine position sets the price ceiling
In a multi-step routine the serum is the step credited with results, while cleanser and moisturiser are treated as supporting infrastructure.
Buyers accept a higher price for the step they believe is doing the work, so the same brand can charge very differently for two products with similar manufacturing costs.
This is why ranges are structured with an inexpensive base and a premium active step, rather than pricing evenly across the routine.
Concentration is not the same as effect
A higher stated percentage of an active does not reliably produce a better outcome, and for several common ingredients it increases irritation without adding benefit.
Delivery matters as much as quantity, since an ingredient that cannot reach the layer where it acts contributes nothing regardless of concentration.
Anyone treating a diagnosed skin condition needs clinical guidance rather than a product comparison, since prescription and cosmetic products work under different rules.