The two closest comparable contracts might be Andrew Hammond and Devan Dubnyk. Even if those situations are close, they are so different. Neither of the two had the post season run. In Dubnyk's case, he was a former blue chip goalie prospect, so maybe there was more comfort in going 6 years.
My guess would be 3 years 4.5 million. 4.5 is about the minimum a starter can get paid. 3 years is short enough that if he is a flash in the pan, the Blues can work around it. It's enough cash that if Binnington never gets another NHL contract, he's set for life. At the same point, if Binnington is the real deal, it sets him up to where he'll only be 29 when the contract expires. 29 is young for a goalie.