Fellow Lions Mike Phillips, James Hook and Lee Byrne have already headed across the Chanel, while Luke Charteris, Adam Jones and Gethin Jenkins could also be gracing the Top 14 next term.
Roberts still has a year-and-a-half left on his Cardiff contract but Blues chairman Peter Thomas admits the Welsh region would be powerless to prevent his departure if Toulouse are willing to buy out the final season of his £200,000 contract.
"I have heard the rumours and it is a major concern," Thomas told the Western Mail.
"Jamie is still under contract for another season. But you know the kind of money that James Hook and Mike Phillips went for.
"When you look at the French clubs and the facilities afforded them by playing in municipal stadiums at virtually no cost, you realise they do have a massive advantage over us. Frankly, no region in Wales can even consider that."
Walescoach Warren Gatland has spoken publicly of his desire to see his top talent remain in the Principality so to lose one of his most influential players would be a big blow.
Roberts was a standout performer at the recent World Cup as he rediscovered the kind of form that saw him named HSBC Player of the Tour for the 2009 Lions.