In British English, and sometimes American English, 'reasonably' is sometimes used as a neutral comparative - meaning that something might not be unreasonable - but still less than ideal. Thus something being 'reasonably expensive' suggests it is merely relatively expensive by comparison, but not necessarily 'unreasonably expensive'.
'reasonably expensive' SHOULD be accepted.
They all kinda mean the same. You can use it all. It is like pretty and quite and kinda. You can say it is pretty expensive. Or it is quite expensive. Or it is kinda expensive. "Behoorlijk" is for me a bit oldfashion. Like I would not use it. I would personally translate "vrij" to "quite" and "best" to "pretty" but that's my own feeling about it
Apparently, the meaning of 'quite" may depend on the stress. In the example sentence, if the adjective is stressed, then it sounds like the restaurant is really expensive. Now, if the stress is on the adverb 'quite', it makes it sound as if the restaurant is moderately expensive. I suppose a lot depends on the tone, the context, the facial expression...
Also, the meaning can vary according to the adjective used (you can find a more detailed explanation here: https://dictionary.cambridge.org/grammar/british-grammar/adverbs-of-degree/quite)