We define a new cost model for the call-by-value lambda-calculus
satisfying the invariance thesis. That is, under the proposed cost
model, Turing machines and the call-by-value lambda-calculus can
simulate each other within a polynomial time overhead. The model only
relies on combinatorial properties of usual beta-reduction, without any
reference to a specific machine or evaluator. In particular, the cost
of a single beta reduction is proportional to the difference between
the size of the redex and the size of the reduct. In this way, the
total cost of normalizing a lambda term will take into account the size
of all intermediate results (as well as the number of steps to normal
form).