how good does the sentence is?

This sentence is grammatically correct and has a classy tone about it.

" and if so, the cost" basically means if one can provide the services, one should let the other party know about the cost.

This is a classy way to simplify the sentence.

I would say it is well drafted.

Please let us know if you can provide the following services, and if so, the cost.

Meaning: Please let us know: (1) if you can provide the following services.
and Please let us know: (2) the cost (if you can provide...)

The sentence structure is one that joins the above two sentences together in a simpler way, while keeping the contents of both. It is a correct form of grammer to add the second part (, and if so) and third part (,the cost) to the first(main) part.

The tone is nice and formal as it preserves the same tone as 'Please let us know.....'.

You are right to ask the question if you are not sure. From your Chinese translation, I don't think that you have any problem understanding the meaning of the sentence.

i think both grammar and the tone are good.

but your title of the question is wrong in grammar:
"how good IS the sentence?" is much better and simpler.