It can be expensive unfortunately
BUT, if you're not too bothered about fading, wipes etc, an idea may be just a video switcher, which allows you to plug in multiple sources and select via a button as to which one you want to broadcast.
The only problem i see here is it lacks a preview output (possibly)
There is a cheap video mixer on ebay at the moment out in Germany, im not sure of where you are from but to europe the postage is ~20 euro. It has a slight problem to begin with, but apparently once it's warmed up it's fine.
Hope this helps