In the long run, you probably won't be saving any money unless you do some heavy repurposing on the GC video. There are some plans in the works by a few different individuals to implement a GC video on a Wii, but nothing solid as of yet. Not that it's extremely complicated, but nobody who is capable of doing it has sat down and spent the time on it.
You probably also won't have much luck going in through the digital port as it's a completely different interface on the chip, unless you rewired the port internally, which would also add a lot of latency to the video lines.
There are a lot more reasons that I could list why you wouldn't want to combine a wii and a gamecube just to save $70 on another board. But in the end it's up to you if you want to do it. I can just tell you it won't be fun implementing it.