Actually, when you consider the man-hours spent in developing concepts, scripts, music, sets, etc., and then production costs such as rehearsal, props, set materials and construction, and then add to that regular payroll for cast and crew, it is very likely that contracting a "ready-made" show is cheaper, and probably by quite a bit.
Of course, as has already been cussed and discussed, you're not necessarily getting a show that meshes with the theme.
I'm not saying they don't have the ability to create any kind of show, just look at Dicken's, Listen to the River, and Headin' West. They seem to do fine on big production shows that are planned for the Opera House, but a lot of the creative vision is supplied by an outside writer/director. And the Echo Hollow show is simply, well, you're standard music show. There is the Saloon show, but it is usually a lot of old bits and music tied together by slightly different storylines; nothing really fresh there.
But the shows with comedy and slapstick that used to be prevalent at SDC are probably quite a stretch for the imaginations of the current staff.