Can someone tell me the cheapest way to measure current.
Cheapest way (ok, maybe not cheaper than inserting the multimeter directly in the circuit, using the 20A current measurement setting most meters come with)?
Homemade current shunt.
Basically, get a thick piece of solid copper wire of a known gauge that can handle your maximum current. Look up for the particular wire gauge what the resistance is per linear measurement. Measure out that much, then mount it using a couple of standoffs (copper bolts/nuts are best) to a non-conductive board (an small HDPE cutting board is perfect); you may have to coil or zig-zag it (don't let the coils touch each other!) to fit it in place. Hook your load up in series with it. Since you know the resistance of the length of wire, you know what it's voltage drop will be, and from that can calculate the current (Ohm's law, remember). Measure the voltage drop across the wire, and Bob's your uncle:http://www.reuk.co.uk/Make-a-Shunt-Resistor.htm
More examples can be found online...
/note that this isn't as accurate as a -real- current shunt, due to heating and other enviromental factors over time, but it is much cheaper