[kps]

Depends on what you consider too expensive, really, whether you have any particular desired features, condition, whether or not you are likely to use it, your patience, and where you are (as different companies served different markets).

The common plastic high-school slide rules still turn up in thrift shops for a few dollars, priced by people who have no idea what they are, or in grab bags with dried-up pens and bent scissors. (The Acu-math on the cover page of the linked article is an example in this class.) On the other hand, particularly ‘collectible’ models like the Pickett N600-ES now sell for comfortably over $100 in any condition.

Personally I'd suggest one of the post-war Japanese-made engineering rules, since they have generally held up well. In the US the Post Versalog is the canonical example, but you can find many similar models under various names (in Canada, Hughes-Owens or Geotec) as well as the manufacturer's (Hemmi).

If you're at all interested in a circular slide rule, they are still available new from the last remaining manufacturer, Concise.

[kps]

Too late to edit (sigh, HN), but although they haven't made slide rules for decades, “Hemmi Slide Rule Co., Ltd.” still retain the name! https://hemmi-inc.co.jp/english/

[spc476]

I was in a small town in North Carolina and stopped off at an antique store. There, I saw two slide rulers for sale, each for $15. I immediately bought both of them, and one of them was a Pickett N600-ES (of the four slide rulers I have [1], it's my favorite despite being the smallest).

[1] Three were bought at antique stores and one at a HAM fest.