What is the required bandwidth, accuracy, CMRR, output impedance, source impedance, signal level, dynamic range, maximum voltage difference between the two grounds? There are tons of potential solutions, but the most suitable one depends a lot on the actual requirements.
A plain differential amplifier, sitting at the output-side potential and measuring the difference between your signal and its ground reference is the simplest, as mentioned. Making it a full instrumentation amplifier might be required if you have special requirements in regards to accuracy or CMRR, or your source impedance is high. Dedicated instrumentation amplifier chips start at around 20 cents. Then there are many more exotic options if you have special requirements, flying capacitor voltage sampling, delta-sigma analog voltage isolators, linearized optocouplers and so on.