Blues have reached agreement with Spurs to pay an initial £4 million for the Republic of Ireland international.
But this could rise by additional £2million via various add-ons and clauses in the deal.
Sources close to Blues insist any potential deal for Keane is still in its infancy.
Blues have been given permission to hold talks with Keane, but the former Leeds player has yet to discuss personal terms with the club.
Blues boss Alex McLeish is a known admirer of Keane, who has been told he can leave White Hart Lane after slipping down the pecking order.
McLeish has continually stressed the need for Birmingham to sign a quality striker. He is aware a lack of goals has seen them become embroiled in a Barclays Premier League relegation battle this season. Midfielder Craig Gardner is their leading marksman with just five goals.
If Keane does move to St Andrew's he will join up with winger David Bentley, who has moved from Spurs on loan for the remainder of the campaign.
Keane spent the second half of last season on loan at Celtic and has had only a bit part role at Spurs this season.
McLeish is still also hopeful of bringing in Rangers striker Kenny Miller in a deal which could rise from an initial ¦pound;600,000 to ¦pound;1.1million.
Miller has also been the subject of an offer from Fiorentina but Blues sources indicate the player is edging nearer to being reunited with his former Scotland manager.