I advise you to go to a Portuguese Real Estate site (with english versions) and try to find properties similar to the one you are interested a see if the prices is overrated or not.

One of the simplest ones is this

BCI Real Estate: Properties, Flats, Villas, Houses, Lands, Plots, Farms, Buildings, Offices, Shops and/or Warehouses for sale or rental in Portugal

Just choose Lisboa and then Cascais and off you go.
By the description and photos you should be able to have some arguments to negotiate.

Maybe you'll even find a better house for a better price!