You can refill the 'ACE' reservoir and drive the vehicle. You will of course need to watch and see how fast the fluid leaks back out. The fluid is a hydraulic oil so you will need to make sure that you put in the correct fluid. The most common leaks I see for this system are from the power steering pump and the hoses.
Depending on what is leaking will determine the cost to fix, and yes the pumps are fairly expensive.
I hope this helps, please let me know if you have any further questions about this.