The most important finding here isn't a price difference — it's an accessibility difference. If you're an engineer looking for a quick price check to compare options, Xometry gave you an answer in seconds. Protolabs redirected you to an RFQ process for the same part.
This doesn't mean Protolabs is worse — they may offer competitive pricing through their RFQ process, and they operate their own factories (which some buyers prefer for quality control). But for quick comparisons and automated workflows, the accessibility gap matters.
Interestingly, Hubs (which Protolabs acquired) did provide an instant quote at ~$152 USD. So within the Protolabs family, there is an instant-quoting option — it just runs through the Hubs brand with a global supplier network model rather than Protolabs' own factories.