There's been a couple of students (in the WGU discussion boards) who've said they've finished WGU's RN-to-BSN program in 3 months.
WGU students pay a set tuition rate ($3,250) per term (composed of 6 months) no matter how many competency units they take. If students finish in 3 months, then they pay the $3,250. I don't know another university that compares.
I guess this depends on how motivated you are to finish the competency units.
The competency units are pass/fail, and students earn a 3.0 GPA when they graduate.