I think a replacement TAD 4003 diaphragm is around $2k. Their cost is obviously lower.
The 950PB with a truextent Be diaphragm is the "poor mans" version at around $1000-1200. I've been told they sound better than JBL's 476Be which, when it was available, was around $3700. I haven't heard of anyone comparing them to TAD 4003's.
FWW, there are guys like Geddes who says he tested the TAD 2001 (the only 1" Be driver I know of) and said it provided nothing over the DE250.
Without a doubt, the BMS is a far better value and I wouldn't be surprised if it rivaled the Be diaphragms in performance.