At a very minimum then that's 2xSGD4,000 = S$8,000 = USD6,300???
Paying USD1000/yr off the USD6300 amounts to a 16% 'tax' on income but of course if you make three invoices each at S$6,000 instead then 'tax' (otherwise, cost) comes down to 7%.
The more you work, the higher you are able to charge then obviously the lower the cost but that's the maths you need to work out for what you can generate and how you think it works for you. At the very least it is a legal, contractually correct business.