After the initial buy in with the soil, there isn't much I have to do. The soil doesn't go bad, it just needs to be topped with compost once in a while, fertilized properly prior to each new crop, and then recharged from time to time (like every 2-3 years). One thing I really like to feed the soil is if I harvest a full bed of lettuce, I'll cut all stalks into little bits, and tear up the greens that I won't be eating, then just bury them in the upper 3-4" of the planters and let the worms and bioactivity break them down a bit prior to replanting any given planter. I don't wait until they break down completely because they are still giving off nitrogen as they break down. I also use Espoma's Garden Tone prior to each planting. I take a couple handfuls and just work that into the top 2" of soil, then plant directly in. It isn't too expensive for organic fertilizer if you buy the larger bags (they are just under 30lbs). As soil levels drop over time I add cow manure compost and toss that into the top part of the soil (you don't want a thick layer on top of the soil because it's actually really hydrophobic once it's dry, and new plants really need that water in the upper column of the planters). When they need a recharge I mix in new soil with the used soil, but it isn't an issue of removing any existing soil, it's just adding new and mixing it in. This isn't needed all that often.
I don't think about being ahead or breaking even, so that would be difficult to calculate. If I were selling my lettuce instead of giving it away to friends and family I'd be wayyyyyy ahead, but that's not my business. I just pretty much never buy what I grow, unless we're talking peppers in the off season. I will say I eat TONS MORE vegetables than before I started growing my own food, that that's something I also wouldn't be able to calculate.