There used to be some relatively cheap flights from Salt Lake City to Houston, both via United & Southwest, so if you could get from Boise to SLC, relatively cheaply, that's one option. The other would be through Denver. The figure of $500 round trip sounds about right, though. One way to cut down costs is to stay over a Saturday night and see if flying one day earlier or later changes the price, if that will work for you. Sometimes that one day difference can change the price appreciably.
Typically the best day to buy ticket is on Tuesday afternoon, so I'd check flight costs then. I'm not aware of any seasonal or "non-peak" fares into or out of Houston, but possibly you may find a cheaper fare from time to time.