A station typically runs from $150 Million to $400 Million, depending on circumstances. Let's try to look at each of the proposed stations objectively.
Lawrence East: This station would be among the most geologically challenging to build in Toronto's history. Its cost should be at the upper range of that $150 Million to $400 Million range. This location isn't particularly lucrative, so I don't expect development rights to be sold here. Also, I don't believe the TTC has made any provisions for the construction of this station.
Sheppard East: This station would necessitate a 1.5 km extension, north from SCC, so we're already in the hole at least $300 Million. This tunnel segment is the most challenging of the proposed SSE route, because it needs to go under Highway 401. $500 Million for this station seems like an extremely optimistic pricing estimation.
All things considered, $1 Billion is a reasonable estimation of the costs for adding in these two stations.