Pay per day as in charging a credit card each day?
It is likely the credit card will flag your company as a scam. No one wants to see their credit cards charged on a day-by-day basis, either, and I think you'd be covering those fees (2.5% + 0.30 for Paypal and Stripe).
I've gone into Walmart, forgotten to buy something, had to go back, and while the second time usually goes through, I get declined after that. I've driven across the country and instead of filling my gas tank up half way or when it gets to E, I've attempted to fill it up every 20 miles or so, even every time I saw a gas station, and my credit card got flagged and I had to wait 24 hours to for them to unblock it.
I know we're talking in terms of minutes to hours here, but "day-by-day" with a charge for the same company may get flagged.
I've been charging on a monthly basis (which Stripe can do automatically) and I've even seen people doing 3-6-9-12 month basis, but there is another method that I'm planning to use, which I haven't seen much. While Stripe doesn't officially support it, there are methods to actually get it to work which allows you to "pay as you go." (PayGo)
In this way, at the end of the month (simply grab the 1st day of the month and subtract by 1 or grab the last day of the month at 11:59:59 PM) and you calculate how much a person used for the entire month. Then you would just change the invoice to reflect the new charges. In my future projects, I will probably start going this route.
I'd think people wouldn't mind putting in their credit card and then just getting charged for what they used (once per month). It is more peace of mind too...
Saas is like: use it or lose it.
Pay as you go is... use it or don't.
As far as my method of building web apps go: I offer X amount of something for free. After that a month, it returns back to the original X and as the person uses it, it counts down until there are no more, and then you have to wait a month to use more of it. If you want more, I offer 2 solutions: subscribe for more or refill your X on the "free" plan which just gives you more without the added benefits of the higher plans.