If I RSP SGD100 monthly into S&P500 through FSMOne, are the costs too high? What are the fees involved and is it worth?

You could do it all by Yourself by investing manually with the cheapest broker You can get (f.ex. "prepaid accounts"). Also it seems nice to invest monthly, but quarterly with 400 SGD is completely o.k. to decrease relative trading fees. Even semiannually would be o.k.

Look also to have no or minimal account management fees per counter, ideally 0.00 SGD.