Both "nunca" and "siempre" can take both conjugations.
For me, the difference is rather the idea of a repeated habit (imperfect) or a one time action (preterite); which is, sometimes, a notion mostly in the mind of the speaker. If the speaker situates him/herself in a repeated action in the past, then they'll use imperfect, but if there is some idea of a time definitely past and with some kind of "block" of homogeneous events in the past, then they'll use preterite.
- Nunca fui feliz cuando viví en Italia.
I was never happy when I lived in Italy.
- Cuando viví en Italia, nunca visité Milán.
When I lived in Italy, I never visited Milan.
- Cuando vivía en Italia, nunca salía de paseo.
When I lived in Italy, I never went out for a hike.
- Casi nunca tenía visitas cuando vivía en Italia.
I almost never had guests when I lived in Italy.
- El abuelo siempre nos regalaba juguetes.
Grandfather always gave us toys. (The speaker has in mind the habit to get toys from grandpa.)
- El abuelo siempre nos regaló juguetes.
Grandfather always gave us toys. (The speaker has in mind the time past when grandfather had toys to give.)
- Siempre era agradable ir a la playa.
It used to be nice going to the beach. (Habit in the past. There were certain conditions --that the listener may or may not know-- in which going to the beach was nice.)
- Siempre fue agradable ir a la playa.
It was always nice going to the beach. (Despite the "siempre", in the speaker mind, it's a time that has a specific start and end. It was nice in a moment and never again.)
Your last example "he never had money to pay (during the month)" is very well applied too.
