driving from lisbon to barcelona

What do you mean an economical way to cross the border? Do you mean renting the car in one country and dropping it off in another?

It's very unlikely that you will be able to avoid a very steep "drop off fee" for renting in Portugal and dropping the car in Spain. The only way I found to beat that was to actually rent two cars: one for Portugal, and another for Spain. Drop the first car at a town on the Portuguese side of the border, get across the border to Spain by bus, taxi or whatever, then pick up your "Spanish" car in the first town across the border. It's definitely an extra hassle, but it saved me something like 400 Euro (and I was determined to overcome the challenge). Just choose your two towns on either side of the border wisely (so you can make a fairly easy connection between them) and sit back and think of ways to spend that extra 400 Euro. By the way - do you really want to drive all the way to Barcelona? For most folks, it makes a lot more sense to take a train (or fly) from Madrid or thereabouts to Barcelona. Just something to consider.

If you'll need the car for 2 weeks or more, look into lease/buyback. You will still have dropoff fees, but they'll be less than what rental companies charge. For example, for Lisbon to Barcelona, Hertz would hit you for a 1500 euro drop off fee (not a misprint), Renault Eurodrive would hit you for about 360 euros. (Eurodrive has a pickup fee for Lisbon & a drop off fee for Barcelona - they total to about 360).

If driving in the route, I strongly suggest a longer route via Grenada that the direct route. That regions is absolutely beautiful, even if you don't leave the highways.