The ESP32 boards I have were £6, including postage. Can't find a Pi Zero for less than double that when including postage costs.
They're designed for different things though - if you wanted a battery powered version that updated on a button press else slept, ESP, if you want live updates all the time, Pi is fine!
ESP32 uses less power. When you use deep-sleep, you can draw as low as 40microAmps, a sensor I've build on a ESP32 has been running for 3 months on the same battery and I expect one more out of it. The battery is a 2200mA LiPo 18650, so nothing that extraordinary either.
The Pi0W probably won't run a day on the same battery.
Though it's important to keep in mind that deep sleep can fairly long, if you use external triggers, you can go days or months in inactivity, which saves A LOT of power.
Though I do agree it depends on your use case, if you're plugged into the wall, a Pi0 might be the better option.
Dev boards, or modules to solder onto a circuit board? If you are printing your own board, then yes, the ESP32 is way cheaper. If you are just making a one-off, though, I'm not sure where it wins except in power consumption.
this are dev boards. Just look here for example: https://www.aliexpress.com/item/32656775273.html, the nodemcu dev with USB for 3,55. You can even get a fully working board with a webcam for under 5 bucks (though that does need a UART bridge, no usb included). Unmounted esp32 chips are going for as low as 2,10
I much prefer using an ESP32 for something like this because I don't have to deal with installing distro's, updates, etc.