I build speakers also and relative to the enclosure, drivers are inexpensive. The driver market is huge and there is lots of competition. Take a Big Wilson. All The drivers might cost say $10,000, probably a lot less. However, the speaker pair retails for $350,000. For the sake of argument let's break this into 3rds, the actual cost of the speaker including labor and associated costs, Wilson's profit and lastly dealer's profit. That would be 117,000 each. The actual cost of the speaker minus drivers would be $107, 000
My point is that relative to making a speaker enclosure the cost of drivers is minimal even with a very inflated figure. It is difficult to spend more than $2000.00 for the drivers of a two way system. The best subwoofer driver for my purposes cost $256.00 each. After all the work on those enclosure I am not about to put a second rate drivers in them and I investigated the entire market before putting pencil to paper..
As for crossovers Eric is correct that the active approach is superior in every respect and easy to implement. The active part need not be in the speaker itself. Outboard equipment is seriously superior and electronics companies are starting to get the message. DEQX has a preamp coming out that has a complete 4 way digital crossover which is extremely flexible giving you a choice of filters from 1st to 10th orders, butterworth or L-R. in 1 Hz increments.