theres a few differences...
the electrical gauge uses an electric sender unit to send fuel pressure measure to the gauge... this basically means you can mount the gauge inside the car because there wont be any fuel lines past the firewall... i know that an autometer sport-comp electrical fuel pressure gauge is $190

mechanical fuel pressure gauges must be mounted outside of the vehical.. e.g. mounted in a cup on the hood.. this is significantly cheaper... probably under 100 for the gauge..

now you will need to find the bleeder valve behind the intake manifold. you will need to connect -4an line to this bleeder valve and connect the other end of the line to the gauge.