A FredArray with the MCM tweeter installed. Total cost of the drivers using this tweeter is about $250 for the pair. Add another $150 for crossover parts, speaker terminals, wire, mdf, paint, etc, and a pair can be built for about $400. I never did care for this tweeter in the Bottlehead Straight 8 (too bright), but with the right crossover to eliminate a 4dB 4-7khz peak and a bit of tizziness above 8khz it becomes very engaging. This is the one that will stay in the FredArrays.
Simpler is better, but not when you're trying to coax good sound from cheap drivers. Here's the final crossover. Those large parts that aren't attached to the board are a conjugate filter for the woofer array. It consists of a 10mH iron core inductor, a 16 ohm resistor, and caps totaling 1,000uF, all in series across the woofer terminals. It compensates for a 30+ ohm peak at the tuning frequency and smooths out the bass.