Como dizer "dane-se (sem ser rude)" em inglês

Good choice Matheus, example:
Whatever, I will buy a new car even if it is expensive.
Não importa, comprarei um carro novo mesmo se for caro.
(Linguee)

Another way would be "screw it" - as in "screw it, let's do it."
In fact, whether "whatever" or "screw it" would go with context. And might have other ways to express that...

Now, "screw you!" would be another can of worms altogether!

It´s indeed an interesting question, because there are cases and cases.

Pick, for instance, the following sentence:
And I command you both to be there and I won't take no for an answer.
(E ordeno-vos que estejam presentes, não aceito uma recusa.)
Source: Reverso.

Could be that it was said to two people that don´t get along well, so the speaker would "mean":
"dane-se, não quero nem saber,quero os dois lá. Estou mandando!"

Thus, sometimes "I won´t take a no for an answer" sometimes is a "dane-se", in a more softer way.