I found several slim-wallet-with-money-clip designs here. I was surprised at how much I liked them, but the spring/clip (printed with PLA, even with 100% infill) would always break after only a week or two. Guess I carry too much cash! I whipped out FreeCAD and created a design that spread the stress in the clip over a much larger area, without increasing the size or weight. I also customized the box for the number of cards I generally carry.
This design will easily hold 12 bills (unfolded) and 4-7 cards (ymmv). I've included the FreeCAD source for those who want to make the box larger or smaller.
29April2018: The v2 files have a little more card space (I have 8 cards in mine; it's a nice, snug fit), makes the walls of the case a little thicker (mine old ones would crack after a few weeks) and rounded some of the sharp edges at the base of the clip.
04May2018: I realized that I always keep the card case full, so I don't need the spring clip to hold the cards in, just to keep the bills organized. The v3 files have a spring clip that is flat, so it doesn't descend into the card space. That makes it easier to get bills in and out and makes it less likely to break when I'm flush.
If necessary, rotate it so the closed end is on the build plate so that it will build without supports. Does require that your printer will do some minimal bridging to get the top edge of the window correct.
On my MP Mini I actually used a layer height of 0.0875mm, wall thickness 1mm, infill density 20%, print speed 120mm/s (I'm not very patient). Sliced with Cura.
The top edge of the window might need a little scraping/sanding, depending on how well your printer bridges.
If you stress the clip by putting in the maximum number of cards and bills (you're rich!) then the clip will deform after a while and you won't be able to get away with too many fewer cards, as they will slide out. If you suddenly find you no longer need as many cards, just re-print.
Within limits, you should be able to use your slicer software to scale this model in the thickness dimension to accommodate fewer or more cards.