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Lets say you are a good player and as of right now, you have a 25% of winning an SNG. Now youre deciding whether to make an -EV play and try to double up, in which if you do, your chances of winning the tournament increase to 70%.
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Your example is really bad.
The normal assumption in tournament play is that your probability of winning the tournament is linear. It is the fraction of chips you have. For good players, the probability of winning should be sublinear. Doubling the number of chips you have less than doubles the probability that you win the tournament.
Typically, in tournaments, the value of chips is sublinear because of the prizes for not finishing first. These are particularly important in SNGs. If you double your chips, you rarely double your expected share of second place or third place. Only 20% of the prize pool rewards you for finishing first rather than just the top 2. 60% of the prize pool rewards you for making it into the top 3 rather than just the top 4.
See the
ICM calculator.
Finally, the OP was talking about cash games, not tournaments.