There's no hard-and-fast set of rules on this. But we've got a few ways of looking at your scheme.
1. Crafting (PHB187)/Crafting Magic Items (DMG 128-129). Each of these pursuits allow you to manage your downtime-game to the level of which X are you creating at what times. While crafting mundane items and crafting magical items proceed at different paces (5gp-worth vs. 25 gp-worth each day), in each case we have the following provision: while crafting a [magical] item "you can maintain a modest lifestyle without having to pay 1 gp per day, or a comfortable lifestyle at half the normal cost."
The reasonable read on that is that the PCs labor "earns" them 1gp per day. It's not explicit in the rules, but you could make the argument to your DM that while your work earns you 1gp you could choose to "squeak by" on a semi-modest or even poor lifestyle, pocketing the difference.
2. Practicing a profession (PHB187). But perhaps you're not interested in the exact items you're crafting, you just want to be a smith. The PHB has rules which allow you to abstract your economic activity one layer and practice that profession. At the low end this earns one enough to cover a modest lifestyle (1gp per day), as in the "crafting" rules. But adding a support organization--a guild or, perhaps, your business partners--in your profession allows one to cover a comfortable lifestyle (2gp per day). As in point 1, perhaps there's room to extract profit from living within one's means?
3. Running a business (DMG129). Frankly, though, it sounds like you're capitalists. Big bosses. You own the means of production. You're running a business. Your business has maintenance costs indexed to its size/complexity that you'll have to figure out with your DM. But here's some guidance:
Maintenance Costs (DMG127). I'd estimate that a good half-dozen skilled workers might productively work at your location (based on keyed locations 12 and 15), with some larger number of unskilled workers assisting\$^1\$. This puts your business in the neighborhood of those with 10 gp/day maintenance costs on the low end, squarely in the conversation with businesses requiring 20 and 25 gp/day, and well below those requiring 50+ gp/day. It's assumed that a functional business left alone covers its maintenance costs, but doesn't particularly clear a profit.
Making a Profit. To make a profit on your business you've got to invest some of your time. I'll spare you the details--they're in the DMG--but downtime days spent on your business produce modifiers to a roll for the business's profitability. Assuming a 25 gp/day maintenance-cost business\$^2\$ (see above), your expected gp/day profit ranges from 5 to 39.75 as you devote between 1 and 30 downtime days\$^3\$.
Putting it all together: "what can the PCs expect to earn?"
- If you're crafting a specified item: <1 gp/day, with DM's allowance of living beneath your means and pocketing the difference.
- If you're practicing your smithing: <2 gp/day, with the DM's allowance of living beneath your means and pocketing the difference.
- If you're "running" the business: 5-40 gp/day, assuming a 25 gp/day maintenance assessment on your business.
Epilogue: so why run a business?
Your business is not going to sit there and generate a return for you unless you invest time into it. But it does play the following roles:
- A place to store (monetary) wealth: if you've got a thousand gp burining a hole in your pocket you can keep it in an obvious chest, convert it to easily-stolen gems, or expand your side business.
- A place to spend (temporal) wealth: if you've got downtime you can turn it into cash at your business. See above.
- [meta] Plot hooks.
1 - This ignores the mine, focusing on the spellforge. As mentioned in note 2, including a mining operation will actually drive down the profitability of your enterprise. But should you wish to include it, the expected returns look like (at 30-day downtime investment) 36gp/day profit for a 100gp/day maintenance operation (ca. 100 workers), 21gp/day profit for a 400gp/day maintenance operation (ca. 300 workers). Diminishing returns, and all....
2 - Not to get too deep into the weeds, but the profitability of businesses diminishes with their size. That's because losses are proportional to maintenance costs, but profits are constant. So it may be incumbent upon an enterprising wealth-manager to consider diversifying business holdings rather than growing, but this'll depend on how you strategically hope to invest your downtime.
3 - The modifier to the business roll plateaus at 30 days.