Here are a couple, untried, offers that can reduce the cost. Pelgao, the Singapore associated travel site, allow you to buy Skybus tickets.
At the moment Amex holders might have a spend $100 get $35 back offer with Pelago.
Pelago allows you to buy 3 x return Skybus tickets for $35 each so $105. That would mean 6 tickets for $70.
I didn't book but went through the process and there are a couple of things to note.
You can only book adult and child tickets, no seniors.
Pelago allows you to select the date of travel but I expect that these work the same way as other Skybus tickets, that is, that they are only valid for 3 months. Unless they send them on the date requested then the 3 months validity will probably start from the date of purchase.
You could always try a date in advance and see what happens and report back.
Otherwise, assume that you would have 6 tickets to use in 3 months.
If you don't have this Amex offer then I also found this 20% offer with Visa that might be worth a go too: