Ok, what to do about this debacle. I ran a small Math and Programming tutoring business on the side during my time in grad school with success. Buuuuut, it could have been better if I'd thought sooner, done the sums, and got on it earlier.
Let's look at the US situation (the same principles apply in many other places of course). If you start tutoring at the start of your Ph.D., for three hours a week (a medium estimate), in groups of two (a medium estimate), for $40 each an hour (a low estimate), for 48 weeks of the year, you will put away 3*2*40*58 = $13920 per year. After 6 years (typical for US grad school), you will have $83520. If you put this into any typical index fund (3% PA is a low estimate) , you will have:
13920*((1.03^6) + (1.03^5) + (1.03^4) + (1.03^3)+ (1.03^2) + (1.03^1)) = $92,741
$92,741 for 3 hrs a week online. Just sayin'.
If you're serious about it and do 6 hours rather than three, you'll have $185,483 and be most of the way to buying a house. Also, just sayin'. More about managing your time in grad school if you choose to do this, especially if you're not resting all your hope on landing an academic position soon. It's worth noting that these numbers apply to postdocs too.