His business is booming probably from other avenues like bulk setup for SME, 30~40 to > 100 pc, with service contract or some retail business which cannot be compared to Sim Lim's volume & etc. Don't think he is making much money or if he is making any money at all by doing house call.
Don't believe, you try to do one yourself & see.
ERP, petrol, truck insurance, road tax & truck's auto loan, labour cost & time spent. Those can only be offset by the hardware he charges you. Since everyone say his price is cheaper than Sim Lim, then I guess it's not much. Say he spent 3 hours per pc including OS install, I think he can do 2 at most 3 pc in a day, not counting the time to get parts etc...
he has been doing for quite some years already
more than 2 years definitely
and is booming definitely a few rigs everyday