I have a zooz zen15 on a lamp I am looking for a low-cost option for a button to turn it on
The Zooz ZEN34 remote isn't unreasonably priced and is on sale this week (buying directly from Zoom at The Smartest House is normally best either way). An added advantage is that you might be able to use Z-Wave Association and bypass the hub for things like on/off.
If you want super-cheap, the Sonoff Zigbee Button is the cheapest I can think of, but some think it's a bit ugly. Lutron Picos remotes are also cheap and super-reliable, but you need the Lutron Smart Bridge Pro or a higher-end system like Ra2 to use them, which will add a lot to your initial cost (but might be worth it if you need a lot of devices or value their appearance or reliability).
I’m pretty happy with my hue button.