I wish they would be quiet
- I wish they were quiet.
- I wish Sarah would come
- I wish Sarah would be here.
- I wish everything wouldn't be so expensive
- 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).