I have no idea how much ANYTHING costs but 250 does seem steep.
Cisco sells the same antenna. Maybe you can find a better price down that route. Make sure to confirm compatibility first.
part number AIR-ANT-SE-WIFI-D=
For the captures i wasn't able to decipher anything but WLAN traffic/signals. I use the same tool but bought it from Cognio before they were acquired by Cisco. I don't know if that makes any difference when reading the captures taken from the AM tool. I will ask in a separate post.
It does seem like the duty cycle is pretty high though. I would try disabling lower data rates on your WLAN to see if that clears up the duty cycle issue.
Have you had any feedback from AM about the captures?