A clamp-on ultrasonic transit time flow meter does not care about what the medium is or what its pressure is, because the flow meter only contacts the outside of the pipe, not the medium directly.
Strap one onto your present piping, connect the power and signal wiring, commission it and get flow readings.
If the line is sufficiently large for an UltraSonic meter then this is the easy way to go. However, if the flow is low and the line size is small, then likely you will need a specialty flow meter such as our MicroFLOW or Series/LFE meters. Both of these are available in suitable materials and have proven successful in this application. Let me know if I can help.
Depending on how close of a tolerance you can live with on the flow rate most flow measurement technologies will work just fine. It is just a matter of getting materials that are compatible.
I recommend you use a magnetic flowmeter - teflon or ceramic lined (teflon is good to about 150-180°C depending on pressure), with platinum electrodes. This will give you an accurate problem free measurement for 20years.
Cost depends on line size and temperature - lots of vendors make them, prices probably from about USD$1000 for 25mm. We use yokogawa, foxboro & E&H in this service from 25mm to 450mm.