What makes an active probe so expensive?
I think low volume and calibration are a big part of it, but basically it's a lot easier to make a passive probe with a tip, a resistor, and a cable be linear and have a flat frequency frequency response to 500MHz than it is to take an active circuit, with little or no negative feedback, have a flat frequency response to multiple GHz.  Also, since the input is usually a high impedance FET, you have to consider how you're going to protect it from overvoltages, static discharge, etc.  The protection circuitry is guaranteed to make the signal path design that much more challenging.