Every time they buy one lab and crush it down, they are losing money. The amount they lose depends on two things:

1 - how much profit the lab owners are making (otherwise, why sell it to DeBeers)

2 - how many labs per year they need to buy

The possible scenarios here are complex. Let's explore one:

"It is cheap to create a new Laboratory, production of artificial diamonds (identical to natural ones) is cheap, and they produce handsome profits because the competing (indistinguishable) product (natural ones) is very expensive"

DeBeers will have a hard time buying all the laboratories popping-up all over the world, while at the same time the price of diamonds starts to drop because of the arrival of these cheap, indistinguishable alternatives.

DeBeers will bleed to death pretty quickly in this scenario.

Please note that we assume here several points:

- artificial diamonds are really indistinguishable

- there is no barrier to creating and producing artificial diamonds, other than the usual business barriers that other industries have (investment, IP, ...)