My attitude on this (and I book my own travel with nobody over my shoulder) is that I don't get to play situational ethics in this case. Either:

(1) I pocket the $1500 and I book the cheapest fare class available and fly however I can fly -- if the cheapest allows a personal upgrade fine, but I won't spend a penny more than I have to.

(2) Buy the J ticket.

I'm not taking the $1500 to fly Y. I'm taking it to save the company the most money possible.

The only variation I'd take is that I'd fly *my* airline. It's my policy without an incentive and this shouldn't change with an incentive.

Of course, I have no such incentive (and of course such an incentive is taxable income) and I always fly Y, so I may not be the best to ask.

Driving 600 miles at 50¢ a mile would return about $300 anyway, though, so the poster who drove could've easily gotten much of his Y ticket money anyway and it probably would be completely above board for any company's expense policy.