。
I'll pay for it.
Literal
I [subject-が] pay [preparatory-っとこう].
払っとこう is the volitional form of 払っとく, which is a casual contraction of 払っておく ('to pay [in advance/for now]'). The preparatory ~ておく adds the nuance of 'go ahead and,' 'for our future benefit.' The volitional ending ~おう expresses the speaker's proposed action: 'I'll pay for this one.'