Why cant we use "bezahlen mehr"?
Here, "wir mussen immer" and "immer mussen wir" can be used interchangeably.
I'd prefer 'Immer müssen wir mehr bezahlen' since 'Wir müssen immer mehr bezahlen' is ambiguous. It could also mean 'We have to pay more and more'.
sollen can also be translated as must