All info taken from camcorderinfo.com
Here are machines currently selling at less than $300, with passthrough capabilities (from what I can tell online):
Sony
didn't find any with passthrough in this price range, surprisingly it was A/V out only
Canon
ZR700 - $250 at butterfly photo
Elura 100, also sub-$300.
Panasonic
also no passthrough at this price point
Well, that was a bit surprising. I thought there would be more options here. Lemme go look at JVC...
hmmm, nothing here either. I'm honestly surprised at the dearth of options. It seems like, if you're buying a camcorder for this (new), that Canon is really your only option. The other of course is to go to a local shop and see if they resell used higher-end models from the other manufacturers, and you could ask directly about passthrough.
One last option: try going on eBay and looking for people selling broken camcorders, higher-end ones with passthrough. These often go for very little money, and generally the passthrough feature is unaffected.
Edit: here's a
Sony which has passthrough, selling cheap on ebay.