South America - Which South American city have the cheapest flights to either Lisbon or Madrid?

I'm planning to go to South America sometime in 2012. I want to visit Rio De Janeiro, Sao Paulo, Mondevideo, and Buenos Aires before flying to either Lisbon or Madrid.

Is Madrid or Lisbon the cloest major European cities to South America? Once I figure out which airlines have the cheapest flights to those cities then I'll try to organize the trip so that I'll go to that South American city last and then fly onwards to Europe.

Thanks for the help in advance?

I just finished preparing my round trip through South America and had a similar problem to yours. An option for you might be Oporto (OPO) in Portugal, TAP has direct flights to Sao Paulo and Rio de Janeiro. Another, although a bit further, would be Rome with Alitalia.
If you're ok with connections, you could try to go via Caracas and then to Funchal (Madeira) on TAP.
Everything else I can think of would go further like Paris, Zurich or Frankfurt

imagine Bogota will probably be the cheapest ex-MAD