Well... yes and no.
One thing to keep in mind is that many "reputable agencies" have employees or outside agents who only get a percentage of the total commission. If you have a $6000.00 AbD trip, for example, the commission in 2008 was 11% at most and for many agencies was 10%. That's a maximum of $660.00. Many agents split that commission right down the middle with their agency. For the sake of argument, let's say that the host agency is very generous and the agent keeps 70% of that commission. (This is highly unusual, by the way.) The agent's share is now down to $462.00. Then there are income taxes on that commission, which in some cases (especially for two-income households) is over 40%. Again, being generous with the numbers, taking out only 30% in taxes brings this to $323.40. Now add in postage, office expenses, 800-number service, advertising and too many other things to list here, and you can see that matching Costco's gift card would have many of the "reputable agencies" working for nothing.
Even if you look at an agency that retains 100% of the commission, you can see that after taxes it doesn't leave much to offer by way of matching. Don't assume that if your agent can't match Costco that it's for lack of wanting to keep you as their client. It's sometimes just not feasible.
Dotti