I want to show potential customers how to compare two different ways to pay for service. One is per employee per month, and the other is per hour. I have set up an excel spreadsheet where they input some variables and it tells them the cost of both and how much they can save, so they can see which model is cheaper for them.

I want to make it nicer to look at though, so when they visit me at my booth at a conference, we can do the math as part of an iPad presentation.

I’d consider just doing it in Numbers; it’s way prettier than Excel so you can get nice graphs and tables on screen together in a presentation style.
This is the Charting Basics template on my iPad.

I second this approach.
Or, to be more practical, the client will have an upper and lower limits for those factors affecting the model. I’d personally fill in those numbers, work out the numbers of both models, and show your client which factor impacts each model and quantify the impact.
So basically prepare all of that in a keynote, attach the numbers sheet in case your client needs to do the math later after the meeting.
But if you are a consultant, your client will appreciate seeing how you provide more visibility on how the models work and change. And that you did the math to help them decide before the meeting, than giving them a calculator to play with.

Try putting it in another sheet inside of the Numbers file, a graph can reference another sheet no problem (though another document entirely is tricky). And this way there’s nothing to scroll over to