The 23-year-old is a target for Forest boss Gary Megson with a bid of £100,000 having been mentioned.
“I’ve got to say £100,000 would probably just about buy his right leg at the moment, considering he’s left-footed," said Swindon boss Onuora.
"That figure is a long way short.
“I had a conversation with Gary Megson not long ago. He said he was interested in him (Rory).
“He was very courteous and kindly said that a few discussions had taken place with directors and chairmen but he almost gave me notice that they did like him and were prepared to offer money for him."
Fallon joined Swindon in November 2003 having started his career at Barnsley. He has scored 24 goals in 87 appearances (38 as a sub) for the Wiltshire club.