I was just wondering, to the customer user would this cost more if the foreign currency was used or their local one given the exchange rate is sometimes very good (e.g paying in US dollars instead of the local currency)
In our experience, the conversion rates most customers get aren't very good, and this will end up cheaper. More importantly, the price can be a sane amount (€14.99/month for a $19.99/month SaaS product) and will be predictable in advance. But you're right; there will sometimes be people who would be better off paying in dollars.
Sometimes is the opposite. In Argentina, for example, it is better to pay in Pesos, because if I pay in dollars, I have to pay 1) the local bank conversion rate, and 2) the conversino rate at the end of the month, which may be higher.
plus retentions that might eventually be used to pay other taxes (or not)