Flights from Glasgow to Stornoway (GLA > SYY) aren't too prohibitive if bought well in advance, coming in at less that 100. The Oban route is arduous, and not to be recommended, with the first boat trip lasting over 5 hours!!! It beats even Ullapool to Stornoway at 2.45 hours.
There's a Dallas, New York, Glasgow route, as well. It just depends how far in advance you book, season and which route you choose, as to prices. There are some deals to be had.
I'm taking the next exam in the last series for my Masters in Inverness; popping down to Glasgow; then catching a flight to Dallas via London. Its set me back approximately $800, which isn't at all bad for May.