Harsh? Not a chance. If you want to hand her the world on a silver platter, be my guest. If you want to challenge her, make her work for every little thing. Epic level characters do not live in a vacuum. There should be (and are in FR there are) epic NPC's all around. Who's to say that the merchant's guild hasn't got one or two on their side. A face-to-face battle over profits is very realistic. The fact that you say she will win a physical confrontation hands down just means you have not developed peers for her. Contemporaries in power and magic who do not always agree with what she thinks should happen will be lurking everywhere.
Being made of money doesn't automatically mean your business works fine. Throwing money at a problem seldom fixes it. Your "experts" will all have their own agenda. These agendas usually interfere with profits. Unless she plans on casting geas on everyone who works for her, she
will get screwed in business. There are no fool-proof plans. Hiring the best, usually means you get forced out of the business because you don't understand everything that is going on. The best, by the way, does not have anything to do with alignment. The "best" foreman, who is also CE can cause a lot of problems: both with profits and with her reputation.
I take it you have challenged the party with epic level monsters? What about an epic level business rival? Now, there's a challenge. If economics and politics is the way you want to go: good; but realize that there should be more powerful forces out there just waiting to chew up a new business. Both direct and indirect threats to the business should crop up all the time. If her hirllings are dealing with the day-to-day problems and she just jumps in to handle the big ones; the hirelings are going to feel (rightly or wrongly) they deserve more. They have many options for getting more (embezzlement, theft, ask for raise, creative bookkeeping, selling info to rivals, etc.).
Return on Investment:
1d100:
1-2: Catastrophe
3-5: Loss
6-7: No Profit
8-99: Profit
100: Double profits
This is unrealistic, 5% chance of a loss when she ignores it most of the time? 93% chance of profit? Are you playing a game where loss is possible? If this is going to be a source of income, you need to make her work for it. Money does not fall out of the sky.
Epic level fighters can miss distressingly often in battle; but neophyte business-owners always make a profit? How often does an epic wizard miss his spell penetration roll? If he doesn't, then you are not challenging your party. The same rule applies here.
Again, it is your game, play it how you like. Remember, challenges make the game fun. If the party is not challenged, there is no fun.
Dave