First of all I think its neat when a character wants to own and operate a tavern/shop whatever.
Here are some rules I made for Star Wars RPG that could fit into D&D
I used Profession and diplomacy checks.
Next I ruled the character invests so much money this month, what is the return. I had a minimum of 500gp per month. You could change this.
So here we go with our fictional tavern, The Happy Harpy owned by Grog the Half Orc.
Grog has a 10 Wis and an 8 Cha. Profession Tavernkeeper 5 Ranks Total skill 5. Diplomacy 2 ranks Total skill 0.
Grog owns the building. Invest 1000 gp for the month.
Now the modifiers. How much time did Grog spend with the tavern this month.
Time Modifier
0 Days -10
1-10 Days -5
11-20 Days 0
21- 25 Days +2
26+ +5
Diplomacy Check. I used this for how much you spent on food, repairs, other stuff that pops up during a month. This does not include wages.
Diplomacy Roll. Add time modifier to roll. These percentiles will be added to the GP return value at the End.
DC 5 or lower -25%
DC 6 to 9 - 10%
DC 10-15 -5%
DC 16-20 0%
DC 21-25 +2%
DC 26+ +5%
Profession Check. This how well you ran the business. Add time modifiers to the end
DC 5 or lower -25%
DC 6 to 9 -10%
DC 10-15 -5%
DC 16-20 0%
DC 21-25 +2%
DC 26+ +5%
Next modifier
Total % from Profession and diplomacy check.
If Negative % modifier -2% from the total
If 0 0%
If positive modifier +2%
So with my quick and dirty shop rules the best you could make is 17% on your investment per month. I made it so the group would still have to adventure. When they retire they could devote full time to there endevour. I figured the fighters would sit for a month running the bar while the wizard is scribing spells and making magic items.
For Grog, he spends a 14 days with his Tavern (-5%). This is his first month owning the tavern (0%). His diplomacy check 16 (0%)
Profession 22 (+2%). Total modifier is -3%. Grog loses about 30 gold this month and gets only 970 GP back. Next month will be harder to make some money, with the tavern (-2%). Well at least the Crypts of Zorkin the Dark are rumored to be filled with gold. Hopefully he will make money there.
These numbers can be changed to what ever you want. Also add modifiers you think apply. I made these so the group CAN make money but won't want to do it full time. This is a hobby for them in a way. They are adventures after all.