These sentences are so called impersonal constructions. Impersonal constructions in Russian are sentences that don't have a grammatical subject. Such constructions often describe emotional or physical states, for example:
Ему холодно - He is cold,
Здесь скучно - It's boring here,
Мне нужно идти - I need/I have to go
But we may also have impersonal constructions of the following type: state (modal meaning) + infinitive.
In your examples we have modal meaning (можно / нужно) + infinitive (заплатить).
In such sentences it is not important, who performs the action, only the state (modal meaning) or the action (verb) are important. That's why it is possible to omit here the pronouns.
But you still may use them and say: Где мне можно заплатить / Сколько мне нужно заплатить.
Without the pronounces the meaning will be the same.
But "мне можно" and "мне нужно" have different meanings.
Мне можно - I may do something.
Мне нужно - I need to do something.