considerably

[jamtin]

hi,

please would someone let me know if this make sense in French?

"les ventes dans ce secteur ont dégringolé incroyablement de 30,4 pour cent"

I'm trying to convey the idea that the sales fell by a large amount (30,4%)

thanks!

[Odrizzt]

The term dégringolé implies a large fell already, so for me no need to say incroyablement.
Anyway, you can say it the way you want ^^

You can also say chuté incroyablement/considérablement.

[jamtin]

brilliant, thanks for the help!

[DearPrudence]

I agree with Odrizzt.
Another possibility maybe:
"les ventes dans ce secteur ont connu une chute consiérable de 30,4%"