Exchange rate spread for banks ( sgd to usd) between own account?

The spread is not fixed and can vary from day to day. It can also vary depending on what currency you are switching to, for example the rate for USD to SGD may not be the same as the rate for SGD to USD.

The best way would be to compare between the rates of the services you have access to. Alternatively, some banks/brokers give you full access to the forex markets (e.g. Interactive Brokers) so the rate that they charge is equivalent to the actual rate at the time.