A beaded screen provides the brightest image for a small audience. A lenticular screen is better if the audience fills a wide angle as seen from the screen. Plain white mat board works well, although it isn't as bright as beaded or lenticular screens. The loss of Kodachrome and the advent of digital image reduced the demand for screens, and they are sometimes inexpensive in resale stores.
That's how much I paid for mine. I picked it up from a Salvation Army store. If you want one right away, you'll pay more but if you're willing to wait you can get one used much cheaper. In the meantime, if you have a flat, white wall, that will work for projecting slides too. Just remember what my signature says about putting slides into projector trays.
The beaded glass screens are satisfying and cheap at garage sales. They can stink though and sometimes have ugly stains and rips. Gives a bit of character. The real joy is standing up in front of your friends, adjusting the focus in and out to see how sharp your slides really are.
Let us know where you are located. Maybe someone has a spare screen they can give you. For instance I have a small one that I don't need.
the more expensive silver screens give better reflection -- they're the only ones that work for stereo, too. Thrift stores have them on a regular basis.
I find for slides plain white screens are better, beaded lenticular and silver screens although brighter are far more directional and the viewers need to be sitting directly in front of them to see the maximum image brightness. My own screen is a Reflecta 50"X50"white tripod screen that I've been using for more than thirty years and found ideal.