I wish they would be quiet or - I wish they were quiet. 2. - I wish Sarah would come or - I wish Sarah would be here. 3. - I wish everything wouldn't be so expensive or - I wish everything wasn't so expensive. 1a is correct: useful to describe something immediate - for example, if the neighbours are having a loud party. 1b is correct, but describes a more general 'quietness', for example you might be complaining about the impact of jet aircraft on wildlife. This is a use of the subjunctive (see 3b below) not the past. 2a is correct, hoping for her arrival - maybe she is late, maybe she isn't coming. 2b is wrong: the verb 'wish' is complicated, it can't apply to present states, and 'would be' is present in this case, 'would come' is OK because it is in the future. 3a is wrong, same problem as 2b. 3b is wrong, but frequently used by native speakers, especially in the UK. The 'correct' form uses the subjunctive: 'I wish everything weren't so expensive'. You can be forgiven for being confused; many 'different' verb forms are identical to each other, and even native speakers get confused, especially with the subjunctive; in 2, Maja mistook 'would' (subjunctive) for 'would' (past tense) and in 1b, both Maja and Bill mistook 'were' (subjunctive) for 'were' (past).

1. "I wish they would be quiet" is used when someone is being loud in the moment (present tense) while "I wish they were quiet" is used when speaking in the past tense. 2. "I wish Sarah would come" is correct when speaking in the present tense. "I wish Sarah would be here", is wrong because the sentence in past tense would be "I wish Sarah HAD come", but in this sentence ,in any tense, is wrong because it is mixing he word "would" which in this context can be a a past tense word with a present tense sentence. 3. "I wish everything wouldn't be so expensive", again, this sentence is mixing past and present. It is grammatically wrong. The correct way to say it would be; "I wish everything wasn't so expensive", as you have written.

- I wish they would be quiet （They're not quiet and I don't like the noise. I want them to change their behaviour!） - I wish they were quiet.（they're not quiet, unfortunately）

They are all correct. Would refers to a future desired state. You wish for something to happen in the future. I wish they were quiet is wanting an outcome you didn't get. You are wishing for an outcome that didn't happen it's in the past. Same as wishing things weren't expensive but they are so tough luck.