For now, I'd say make a reasonable effort to address the biggest audio issues without extra charge. The built-in noise reduction tools in Premiere Pro CC and FCPX are OK these days; I'd guess it's the same for several other NLEs. And some EQ may let you fairly quickly notch out some noises. Or invest in iZotope RX. Even the lowest-cost version (which often goes on sale) is pretty helpful: https://www.izotope.com/en/products/repair-and-edit/rx.html
Wow didnt know the Izotope plugin has low cost package, well maybe I will try the element for 199usd and see how it turns out.
That is another thing (to the OP)— Sometimes you have to explain in very simple language why certain more technical things can be a problem ("If we use only available light it might be a problem because blah blah blah”), and like the guy above me said, why/how problems can arise from using only venue PA systems for sound. Things like that. Often our clients don’t have the faintest clue about this stuff that is very technical but makes a huge difference in the final product you’re delivering. Sometimes we have to educate them a little. Sometimes they will even sign off on things or tell you “oh it’ll be fine” and if/when it isn’t you STILL get blamed. I still struggle with this myself because I am not very authoritative. So if you protest about something and are still overruled, and the result turns out poorly, and you are still blamed even though you told them it would turn out poorly, what are you to do? More than likely they will not be apologetic about making bad decsions and may not want to work with you again. So again I say one of the best things is to try to find out about these conflicts or issues beforehand during initial discussions and simply be willing to refuse the job if you don’t think there’s any way it will turn out well.
There is so many good advise from many posters here, thank for all the inputs.
Surely next time I know how to deal with the client better