Well, you don't need any active circuitry to do that. This is all you need for a unipolar automotive current meter with a display range of 0~50 mA.
F1 is a Polyswitch resettable fuse. It's rated for a hold current of 0.4A. Current through it causes it to heat up, and at a certain temperature it quickly transitions to a nearly open circuit.
The time to trip depends on the ratio of current to rated current. The RXEF040's initial resistance is about 0.7 ohms; with a 12V system, this limits the current to about 20A. At 10A or more, the RXEF040 will trip in less than 30 ms.
D1 and D2 are 3A diodes. They will clamp the voltage across them to about 1.0V (positive or negative). They can withstand short overloads much greater than 3A.
R1 is the current shunt. A resistance of 5.6 ohms will drop 0.28V at 50 mA, which is the maximum design current of the circuit. At 0.28V the leakage current in D1 and D2 ought to be a lot less than 50 mA, but the graph in the data sheet only goes down to 1A. If anyone knows what the typical forward and reverse leakage current will be for a 1N5400 at 0.28V, please let me know!
R2 drops this voltage to a value suitable for M1, and provides the calibration adjustment. M1 is an electromechanical (pointer-type) panel meter with a full scale deflection current of 50 µA. The coil resistance is 3000 ohms; I got this value from the specifications for a 50 µA panel meter that's available from Jaycar (an Australian and New Zealand electronics supplier), catalogue number QP5012 (see
http://www.jaycar.com/productView.asp?ID=QP5012). A 3000 ohm resistance at 50 µA has 0.15V across it; R2 drops the 0.28V down to 0.15V.
Simpson Electric (
http://www.simpsonelectric.com) in Wisconsin also make 50 µA panel meters but theirs have a coil resistance of 1800 ohms. A value of 5k for R2 will work with these meters too.
Simpson Electric also make centre-zero analogue meters. A Simpson Electric 25-0-25 µA meter should drop in to that circuit if you want a bipolar indication.
The small components cost under $2. The main cost is the meter movement, the enclosure, and the probes.
The overall circuit has a resistance of about 6 ohms from one probe to the other, and can withstand a direct short across the battery. The polyswitch fuse can take a little while to cool down after an overload though.