how much to secure the border?

About $15 million. That's with an m not a b. 

Concertina Wire Price per Meter - Affordable Security Solutions

The US-Mexico border is about 3,100 km long. At $5 per meter, that works out to about $15 million. That amount is chump change when it comes to federal spending. 

It won't stop everyone, but it will slow down 99% enough to be captured by border patrols. 

Add to the wire anything that rattles, like tin cans. That's an old trick to alert passing patrols.