My company has Logitech R500 or R800 clickers in every conference room. The R500 can be had for 40USD some places and the R800 is still under 100. I'm sure cheaper options are available but I really like these -- they work really well from a pretty good distance and feel good in the hand. The R800 has a programmable timer that vibrates to help keep your pacing during longer presentations.
Generally, they come with a USB dongle and work like a keyboard with a few very specific keys that make PowerPoint do the right things. If you aren't using PowerPoint, be ready for compatibility issues. A lot of these things are built on the assumption that people only use Microsoft products.
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