Tough.

In their favor: Fuel efficiency of newer engines.

The big hurdle: New design airplanes (ex. 175) have a lot of expensive tech, FBW, advanced engines, gucci cockpit stuff. All that extra cost has to be amortized over the life of the plane. Imagine a 50 seat version of the E-Jet... essentially the same cost, minus X amount of structural materials and cabin fittings. But only 2/3 of the revenue potential of a 76-seater.

Would be economical on a few select high dollar markets, but then you have to amortize R&D over a small number of planes, driving unit cost even higher.

Boeing/Embraer will have to come up with some pretty revolutionary mfg processes (BA is actually working on that) to cut costs, or somehow design a very bare-bones jet with all the costly bells and whistles removed.