“The taxi will cost around $20ish.”
It’s fine to put “around”in the sentence.
But why can’t I say “The flower is kind of prettyish.”?
Why is it weird to put “kind of” in the sentence when there is already an -ish word there?
What’s the difference between the two?

See a translation

Really good question! It’s okay to put ‘around’ because we are talking about a range of numbers, you could also do this with time: ‘I will be done around 8ish.’ You can’t say ‘the taxi will be kind of $20ish,’ either. You could get away with saying ‘kind of’ and ‘ish’ in the same sentence but not in a formal setting. The reason you wouldn’t be both is because it’s repetitive.

We already know the flower is ‘kind of’ something because you said it is ‘kind of,’ this is what we hear when you add an ‘ish,’ at the end. It’s like saying ‘the flower is kind of pretty kind of.’ Whereas with the money sentence we hear ‘the taxi will cost around 20 kind of,’ there isn’t that repetition.

I hope that makes sense. Let me know if it doesn’t.