Rough numbers here because BOX hasn't reported full 2017 yet but:
Dropbox 2017: $1107M Revenue
Dropbox 2017 Paying Users: 11M
ARPU: $100 [1]
Box.com (Q4-16 to Q3-17): $479M Revenue
Box.com (Q3-17) Paying Users: 9.7M - 10.2M [2]
ARPU: $47 - $49 [3]
[1] $1107/11
[2] "over 17% of our registered users [57M] were paying users"
[3] 479/10.2 to 479/9.7