It looks like costume, which doesn’t necessarily mean inexpensive. I’m more familiar with the intertwined “YSL” logo but I have seen this one before, for what it’s worth. I don’t know of a costume jewelry forum - and if you find one, please come back and share it with us! - but this seems to be a pretty good site for information about costume jewelry;
They also have some YSL pieces for sale do you can check for other logos... and prices.