One-way trips should be cheaper then two ways. and one pick up cheaper than two pick ups.
Try searching for "tong tar transport" or "Triwins Transports Services Pte Ltd" or "republic express pte ltd"
I have been working for the govt dept/stat boards/schools...so these 3 i have tried before. i think they should be able to accommodate to your needs, but i duno about pricing. mine's govt gebiz price, think slightly cheaper. you shld call them, tell them what you need and ask for their quote.
shld range about $60 (for 20 seater)or ($80) for 40 seater.... (some bus companies ahve 30 seaters too, but might as well get a 40 seater)
for me, since i'm doing it for my company pple, easier as I can estimate number needed.
do check for extra charges during peak hour rate/day.
(Going back, I wun cater bus.... hee hee, i'm more practical, i want them to reach on time...hee heee hee)