pretty much anything i would add has basically been covered except how much you shoot.
about 2-3 years ago I asked myself the same question (cast own or buy). Back then I was shooting about 600 rounds a year out of my then only ML, a .54 flintlock.
I actually did a bunch of calculations to compare economy of buying balls (then about $11 per 100 for 0.530 or 0.535) versus casting. Assuming I could find free lead, I figured with buying a standard (dip-style) Lee electric pot, Lyman single mold and used handles, and dipper, it woule take me 2-3 years to pay off the investment in the equipment.
However, I've since dropped to shooting perhaps only 100-200 rounds per year (many other new obligations have prevented as much range time as I used to enjoy).
But I came in to about 60 pounds of free lead and, very importantly, I started recapturing my balls and am able to collect about 80-90% of lead I shoot merely by shooting into magazines stuffed into cardboard boxes.
Lastly, there are the intangibles mentioned by others: the sheer enjoyment of making your own, particularly gratifying to use your own balls to harvest game!!!
BOTTOM LINE: if you shoot a couple of times per year, have only 1 or 2 round ball MLs, and hunt (and fire) just a few times per year, it's probably NOT worth the investment on a purely monetary basis.
If you shoot >100 and definitely >200 or 300 rounds per year, it will pay for itself ESPECIALLY as lead become more expensive to buy and harder to scrounge.
Like MOST hobbies, economics really aren't part of the equation. Do it because you love it, enjoy it, and don't worry about the nickles and dimes - probably small peanuts compared to mortgage/rent, car payments, grocery bills, etc.