Hadrian's Wall, designed to prevent tribes from invading the civilized south, took six years to build. The Great Wall of China, designed to keep northerners out of China, was constructed over centuries. The Berlin Wall, designed to stop East Germans from fleeing to a more tolerant government, took five years to build.
Independent estimates put the cost of the Mexican Wall at USD 20-25bn. The materials required are approximately three times those used in constructing the Hoover Dam. The main economic impact may be diverting labor and resources from other construction projects to the Wall (raising inflation).
The US dollar did not join the fiscal stimulus-induced equity rally. An increased fiscal deficit increases the current account deficit. The decision to start construction also means that campaign pledges must be taken seriously; pledges on trade protection and Muslim immigration may threaten capital flows into the US.
The Italian courts have altered the electoral law, making early elections possible. However, political uncertainty is not, perhaps, entirely unknown in Italy. UK fourth-quarter GDP growth is moderately interesting for markets, as there are signs of slowing momentum as consumers start to react to the consequences of exiting the EU.