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I should calculate the present value of the future cash flow of SBI and compare it with 0.5 Steem, considering also the risks. (risk: the probability of the unwanted end of SBI flows.)
https://www.investopedia.com/terms/p/presentvalue.asp

But I don't now the cash flow, and the risk either.

hmmmm... ok, cash it is then :P

P.S. Assume the following: SBI = 0.005 per week (indefinitely), and risk of ending is the same as the risk of Steem itself ending)

26 cent a year? (52*0.005. Or 29,6, if we assume reinvestment, 1.005^52-1)
Not bad. At least 26/0.5=52 percent p. a. compared with 0.5 SBD as theoretical investment. (alternative cost)
Incredible.
The question is how is this possible? Is it too good to be true?
High yield bonds can pay also 8-12-14 percent p. a., but in case of companies with financial problems, with very high risks.

Check out @SteemBasicIncome

a) It's not really a 0.005 "return", but rather a 0.005 Steem vote value per week per share... i.e. if you don't post, you don't get the vote.
b) Shares are only "sort of transferable", and can only be sold for less than their purchase price ... or kept indefinitely.

Than the income is 0.005520.75=19.5 cents/year, I suppose.
People pay 0.5 SBD for 0.005 Steem vote value per week?

It's actually 0.5 STEEM (not SBD)... so a little cheaper... but yeah... that's essentially it.

Well, to be 100% correct. The price is 1 STEEM and then you nominate someone else to get the same (i.e. buy-in = 0.5 STEEM x 2)