At the cost of building enough solar and wind farms to supply the nation I would guess, with additional costs to upgrade the distribution infrastructure for smarter load balancing, etc.

It's quite possible Portugal has put out a white paper on their transition toward renewable energy that you can find with some hunting about.

Wikipedia has a rough overview that'll provide some points to drill down further into if you're interested in the per capita comparative costings of a hybrid national power scheme.

https://en.wikipedia.org/wiki/Renewable_energy_in_Portugal

Good question! At what cost do the externalities of fossil fuels come? What is the cost of inaction?

What are you trying to achieve with this banal line of questioning?

On one hand, solar and wind are very cheap - significantly cheaper than fossil fuels. On the other hand, you still also need either a significant amount of fossil fuel capacity or energy storage available to kick in during hours when solar and wind can't provide as much production. So instead of building enough fossil fuel production for 110% of your peak need, you might lower that to 50% and then still also build 110% of your need in renewable energy -- so you'd be over-capitalized at a total installed capacity of fossil fuels + renewables at about 160% of peak need.

So, marginal cost is lower. Total capital outlay might be higher. Portugal provides energy at an average of 9.7¢ / kWh which presumably is high enough to pay back all the capital costs in addition to the marginal costs. Mississippi only has 1% renewables and provides electricity at 11.55¢ / kWh.

So it would appear at a very rough glance that the answer to "At what cost?" is..."Negative cost".

Wind and solar are cheap as chips. Hydro has always been cheaper than alternatives, where the geography is favorable.

Around 9 eurocents per kWh