I assume you're asking about taxi transfer from the ferry or the airport to Bwejuu. I took a taxi from the ferry to a hotel in Jambiani, so I can only speak to that, but I think the prices would be similar to Bwejuu since it's just up the coast. Our hotel arranged a transfer for $60 (three people, one way). I thought this was pretty steep, but it was really nice to walk right past all the annoying people at the ferry aggressively pestering you for a fare. On the way back, we arranged a ride with a taxi who had dropped some other people off at our hotel and we paid $30. He arrived early to pick us up and even took us around the market in Stonetown before dropping us off at the ferry. This was much better deal than the hotel transfer. We also asked him to call a friend in Dar to pick us up there.
I'd recommend this guy to anyone and would use him again, so I'll leave his contact info here. Negotiate your price ahead of time and make sure you'll know how to identify him when he picks you up amid the throngs of other cab drivers (e.g. a sign with your name).
Phone: +255 773 109970
Even if you don't use this guy, I'd highly recommend arranging your rides ahead of time so you don't get sucked into the crowd of drivers with dollar signs in their eyes.