Single pane windows, would shutters be a frugal fix?

Storm windows are a thing in four season climates, basically just another pane of glass over the existing window in winter. I have known low income people to just staple clear plastic over the frame for the same effect. You might give it a try. Ugly, but better than boarding them up.

I don't think shutters will provide much of an R value, but they might block out some sun. Window insulation film is a thing that exists, and might be your best budget bet. Window treatments also work well, and vary in price.

Usually double panes are more important in the cold, not in the warmth. A house is typically kept at around 72F. Depending upon where the house is, seeing temps in the negatives is very possible in the winter. That's a Tdelta of 70 degrees or more. In the summer, your Tdelta is probably not much more than 30 degrees since few places get much higher than 100F or so. The higher the Tdelta, the more heat transfer there will be.

If you have issues with too much heat coming through your windows, then an easy solution to try is tint the windows. Home Depot has window film which you can cut to size and install on any window, including patio doors. The best ones have a reflective coating which bounces the light away from the house. You can install them in just a few minutes and I think most of them are totally removable.

Having shade is incredibly important as well. Planting a tree or bush which will give the window and the side of the house shade will do wonders to cooling a house down. A pergola or other structure that shades the area is an easy solution as well.

Shutters won’t help much. Storm windows are the cheapest help. We just slowly replaced them (exit: the windows with double pain, has filled, vinyl with UV coating) ourselves ($250-300 each at a big box lumber store).

I think shutters (preferably white) would probably help on south and west facing windows. I don't really see them used much down here; the ones I do see are fake. Highly reflective silver tint (applied on the outside) helps more than dark tint. Window awnings would also help. Large trees are the long-term solution :)

Windows are usually towards the end of the list of things to upgrade. What's your insulation situation like? Attic, walls, ductwork? Have you done a leak test to see where conditioned air is escaping? Has your AC been serviced and in good operating order?

I'd check all those out before replacing windows. As other poster suggested. Especially if you have windows that are in good condition otherwise.

FWIW, I’ve heard that if you let vines grow directly on the exterior (or if you run cables and plant the kind of vines that need to climb cable), the vegetation will insulate/shade the house from the extreme sun.

I can’t say it’s 1st-hand knowledge but perhaps worth looking into. Evaporatie cooling might have a small role in that too. Of course the caveat is if you let vines grow directly on the stucco, they will destroy the stucco which many folks consider a bad trade-off. The other caveat is plantlife brings insects so you might not want to bring insects right up to your walls.

I'm using cardboard between the window and screen to block the sun, it has made a huge difference.