jr. members are not much of value i say no more than .005
but yes I guess they can be good business if you have time to make 50 accounts in 1 day and then make 42 posts on each one (2100 total) to get to junior ... then you make .25BTC if you successfully sell all of them
I think you are better off just joining sig campaign if you have time to do 2100 posts, and also the waiting time in each post
If we use your calculations:
it takes 2100 posts to make 50 Jr Member Accounts.
it takes 2x2100=4200 posts to make 50 Full Member Accounts.
It takes ~12x2100=25200 posts to make 50 Legendary Member Accounts.
Current value of legendary is around 0.8 BTC
For 12 months of work with 50 accounts, you would get 40BTC for the year of work!
That's Really Good!
40/25200 = 0.0015873015873015873015873015873btc/post (better than a sig campaign!)