Assuming your problem is the space is getting too hot in the summer and not necessarily too cold in the winter? If so, the problem is more needing to reduce the solar gain/solar heating thru those windows and less about the poor thermal performance from the single pane, possibly laminated glass or just cheap dual pane glass in the windows.
Cheapest, fastest solution is to get a solar controlling film (aka tint them) to reduce the amount of solar energy that comes in. You can get a DIY tint/film kit or there are likely several companies who will do it for your. I'm in the window business but this isn't part of what I do, but when I talk with our sales team/distributors the most common manufacturer of tints/film they work with is 3M. That's just who makes the product, not who is installing it, but I don't think you'll have an issue finding a local company to do this for a fraction of the cost.