a 10 euro al mese = from 10 euros a month
per sempre = for ever.
Remember all the different meanings that a can have. Let's look at it when it means 'per' as in 'per month' (often translated as '... a month'.
al mese = per month
al giorno = per day
all'anno = per year
alla settimana = per week
If a comes before il as in il giorno, then a + il becomes al
If before la as in la settimana then a + la becomes alla
If before l' as in l'anno then a + l' becomes all'
If before lo as in lo stadio then a + lo becomes allo. In this context, though, a means 'to'. I just couldn't think of an example for 'per' with a lo word. You might suggest one.
And those are just the singular options! But you can see a pattern forming.