I don't know the answer, but here's a guess:
The only dropping resistor value I have heard mentioned in probes is 1000 Megohms. So, if the built in resistor is 1000 Megohms and the meter built into the probe is a ammeter with very small resistance compared to 1000 Megohms, 50 kV full scale would be 50 microamps. (50x10^3/10^9 = 50x10^-6)
On the other hand, this meterless probe with 1000 Megohms is intended to feed a 10 Megohm-input VOLTmeter for a 100:1 scaling:
http://www.fluke.com/fluke/usen/Acce....htm?PID=55356
Edit: found this page that says the BK probe is 600 Megohms:
http://www.bkprecision.com/products/..._datasheet.pdf