Can't be of much help, as I'm not really up on the current prices. On Etsy someone is charging £ 11.30 a kilo for logs, though I'm not sure how dry this is, There is also a guy on eBay, at the moment, selling pieces for chess sets and flutes who seems to be, asking good money for his stuff. However, his flute stock will be what is known as music-grade, which comes at a premium. Though it might be worth a look. as a guide to pricing.
In the end, I suppose, it is all down to the quality , number if splits and knots etc. I'm sure that if you could put some time into prepping and grading it you could certainly up the price you get
I might be interested myself in some pieces that I could make whistles out of . I usually purchase these as square billets 28 x 28 x 305 mm. In Blackwood these are about £5.00 for A grade, and £ 8.90 for music grade. Not sure exactly sure how this compares with boxwood prices, though my gut feeling is that they will at least be equivalent.