Multi-monitor workstations are a surefire way to improve your productivity, whether you're using one of the newest Macs or a great PC. You can simply fit more on the screens, saving you time and energy that you would've spent switching between apps, windows, and desktops. Unfortunately, high-quality monitors are still relatively expensive. If you want two or three of them at your desk, the bill can rack up quickly. Ever since I bought an M3 iMac for myself, I've been craving a bit more screen real estate to add to the inbuilt 24-inch panel.

Finding the right external monitor has proven tricky. I need something that's affordable, high quality, fits into my desk setup theme, and has excellent macOS display scaling. That last point is usually where displays not made by Apple usually fall short, excluding a few models from LG. I couldn't bear the thought of spending $1,600 on the Apple Studio Display, either. It meets all my needs on paper, but the price tag is absolutely preposterous for the feature set it offers — it even lacks local dimming zones.

Nearly a full year later, I think I can stop my search. I found a slim and sleek 1080p monitor that meets all of my needs for under $100. Oh, and it just happens to have a computer of its own inside. It's another iMac, and I'm here to tell you why decades-old models could be the best value external monitors in 2024.

Wait, you can buy a working iMac for $90?

Yes, and I was surprised to learn it too — here's why

I keep a keen eye on the resale value of old Mac computers, but even I was shocked when I saw you could buy a working used iMac on eBay for well under $100. The one I ended up purchasing totaled about $90 after taxes and shipping, and it was for a 21.5-inch iMac released in late 2012. This computer came with 8GB of DDR3 memory, a 2.7-GHz quad-core Intel Core i5 processor, and an Nvidia GeForce GT 640M graphics card with 512MB of vRAM.

In the screenshot above, you'll see a listing for a similar iMac that's even cheaper. Depending on what condition or model year iMac you're willing to buy, prices will vary, but eBay is flooded with models under $100.

Why is that? It largely has to do with how these iMacs were deployed. They were popular for home use, sure, but they were abundantly used as enterprise workstations in schools and businesses. Now that these iMac models are obsolete, they are liquidated in large quantities at low prices — some businesses and schools even give them away. They make their way to the hands of resellers, who flip them for a slight profit, and the rest is history.

As a result, you might think that these computers are beat up, but that's only somewhat true. The one I ordered on eBay came downright filthy, and had an ancient version of Mac OS X installed. However, after about 20 minutes of wiping it down with multipurpose spray and display cleaning solution, it looked pretty fantastic. I was able to officially install macOS Catalina on it — not bad for a 2012 computer — and you could even go newer if you're willing to use the OpenCore Legacy Patcher.

By the time I had finished these simple steps, I was extremely pleased with the results. This system is limited by its spinning hard drive and its older RAM, but it's still usable even as an independent computer. With a few upgrades, you could easily use this machine for light productivity work. It's relatively easy to swap the hard drive for an SSD, or it's even easier to use an external SSD as your Mac's boot drive.

How to use an old iMac as an external monitor

Various third-party apps are available, but I chose Duet

Apple offers two first-party software solutions for using an iMac as an external monitor: Target Display Mode and AirPlay to Mac. However, these features only work with specific models and OS versions. Neither works to bridge an Apple silicon Mac to an Intel Mac, which means we need to use third-party solutions. Luna Display sells a physical dongle as a one-time purchase in lieu of a subscription, while the cheapest Duet Display plan costs $4 per month — but it's completely wireless.

I went with Duet, largely because it's easy to try out with a one-week free trial. Your experience using Duet Display wirelessly can depend on the speed of your network, and the trial is a good way to gauge latency before committing to a subscription. Plus, I've been using Duet in some form or another — it was used for Sidecar before Sidecar existed — for a decade.

I thought about using Luna Display, or even going as far as to hardwire an HDMI input to my iMac's display panel with an aftermarket converter board. However, the ease of use that came with Duet Display made it the most attractive option. You simply need to load up the Duet app on both computers, connect them to the same network, sign up for the free trial, and select the Extend displays option in the app. After tinkering with the resolution and display scaling, you're off to the races.

With Duet, you have the option of using a wired or wireless connection, but wireless was more than serviceable on my 1GbE network. There was minimal latency, and it felt similar to using Mac Virtual Display on Apple Vision Pro or Sidecar on iPad. Put another way, Duet feels like a first-party solution. If I had to call out one complaint, it's that the color accuracy of the secondary iMac screen took a hit while using Duet.

The case for using an old iMac as a screen

They're a cheap way to get a high-quality display — especially if you pay more than I did

There's a real reason you might want to pick up an iMac of your own for use as an external monitor. While I used a 1080p, 21.5-inch iMac with a relatively unimpressive panel, this method will also work just fine with a 5K Retina iMac. Those 27-inch panels are gorgeous even by today's standards, and in fact, they're still used in the Studio Display. A 5K iMac will cost more than the $90 one I picked up, but they'll still be available at a fraction of the cost of a new 5K display.

I just might head to eBay and order a used 27-inch, 5K iMac myself. Even with the $48 annual subscription, Duet Display paired with an old iMac gets you impressively close to the Studio Display experience while saving you over $1,000. Since the Duet app is cross-platform and works on Mac, PC, iOS, and Android, I'd recommend this project to just about anyone looking for a cheap and quality external monitor.