What I would do is research the going rate for home child care in your area. Just check online and in your neighborhood.
That seems crazy to me. That's what we paid for childcare per child per day for a preschooler over a decade ago at a facility.
Keep in mind, it depends on what you offer. I think some parents would pay more for structured environment, outings, pre-school environment, nutritious meals, etc.
Not trying to be critical - but some outfits are set up as actual pre-school type places, so I think they charge more and the moms running them are sometimes trained.
So - when you check rates to compare, I would just be mindful of that.
I would think check per child rate, and then usually they don't charge double necessarily for 2 kids - but babies usually run more to care for (they did at our daycares).
I think 3 year olds do get into things and make messes - that part can be expected. As for rules, etc. that's pretty standard.
Remember she doesn't get to set the rate - it's not what "she wants" rather what you ask. You can compromise, but it should be close. If it's not, then it's not the right fit and you should find some new kids to care for if this is what you want to do. Good luck :)