I have long since thought it was narrow minded of PC manufacturers to not have at LEAST an RGB SCART input on VIVO/TV Tuner cards. More interesting perhaps would be a VGA input so you can capture the output of one PC on the other one, or in this case output from the scandoubler on the Amiga.
If someone DID create a VGA input card for a PC they could even support none standard rates so it could become a scandoubler itself. Its not as if scandoublers are a complicated technology, its just there isnt a huge market for them so they cost a lot.
The closest I have seen is this
http://www.cwcembedded.com/products/0/4/312.html which im betting DOES cost a lot. :-o