I do not know where you got the idea that tolls and ailing economy is linked. Tolls have been around for far longer then the present financial crisis.
All what non-Portuguese would call highways/motorways are tolled.
Some have manned tollbooths other you only have to drive under gantries and it registers you.
If you have a Spanish hire car then you will need to stop and purchase a ticket to prove you have paid your fees.
All can be read here:
http://slowtalk.com/groupee/forums/a/tpc/f/5031043523/m/789006232001
in the past some posters on this forum have encourage other travellers to beak the law and not pay the tolls. If you choose to do that it is your choice.

The"N" roads in Portugal are not tolled. However the journey times are a lot slower.

Thanks Riberiasacra,

I'll be travelling in a friend's car, not rental. The economy thing is just what I was told but happy to forget that. Happy also to not break the law, and to pay at manned tollbooths, it was just the driving under gantries and being registered I was wondering about - how/where do we pay that?

I post all the information in one place now as it saves me repeating everything.
http://slowtalk.com/groupee/forums/a/tpc/f/5031043523/m/789006232001
Pay either at a Post office or at a service station on the roads where the electronic toll roads have been implemented.
To plan routes I either use Google maps or viaMichelin. Neither show the electronic toll roads!
If you are using your friend's car ensure you have the correct insurance and you carry the required documentation and safety equipment. They will obviously tell you what you need.
Have a great trip.