Mike,

It would definitely be worth it if the foot traffic was there and you geared the booth and displays towards advertising your products and services, rather than selling engraved trinkets. Think of it as the equivilent of placing an ad in the newspaper. In reality, if you didn't sell a single item, the both cost would be less than an ad in the newspaper, plus you would get face-to-face exposure.

I would create a mini showroom that would display some of the best examples of your work. I'd spend all of my time out in the aisle talking to people, passing out cards and flyers, telling them about what you do and asking them if they use engraving services. Hand out a unique, functional promotional product to those who are willing to stop and listen.