It'll depend on your usage pattern and local cost of electricity-- if you're running that device that uses 10W more power 24/7, that works out to 87.6 kWh of additional power usage over the course of a year, or about $15 at the current average cost of electricity in the US ($0.17/kWh as of June 2023). If that's a monitor, you'd have to run it for a pretty long time before your break-even point on purchase cost alone (10+ years on a lower-end monitor, and probably not in your lifetime on something from Apple).

If your usage isn't continuous (which it probably isn't), that'll reduce your electricity cost and extend your break-even point even further; if electricity costs more in your region, it'll shorten it (average EU energy prices are around double that of the US IIRC, and some countries are much higher). But it's still going to take a while to break even if the efficiency improvements you're considering are in the 5-10W ballpark; that's not all that much power in the grand scheme of things.