There are several levels of slide scanner.
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Below $100, I have never seen a good review and would avoid them.
For $100—$150, a flat bed scanner with built-in slide illumination in the lid can do a very good job. I can recommend the Epson Perfection Photo 4490, and the Epson Perfection V300 Photo gets very good reviews. But, as @arpinum writes, it is very time consuming.
@grumpyfish‘s eBay suggestion looks like a good way to move up, buying used equipment and reselling. You could get a stack feeder, which is much faster than a flat bed.
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If all that sounds like fun, go for it. If your eyes are glazing over, scancafe.com sounds like a great deal.
I am slowly scanning a family collection of slides from the 1950s. Here is an example from the Epson Perfection Photo 4490. The original scan is about 5 times bigger, but you can get an idea of the quality. That is from an Epson 4490.
The $500+ scanners like DarkScribe uses will do even better (my original slide has subtle cloud detail which was lost in the scan).