Prices there tend to reflect newer housing stock and a different mix of buyers, many trading a shorter commute for a lower price point, but the specific number for any given house still comes from what comparable Martensville homes actually sold for, not from scaling the Saskatoon figure up or down.
Property tax works differently too. Martensville has its own Board of Revision for assessment appeals, separate from the City of Saskatoon's, so a Saskatoon buyer moving north needs to budget against Martensville's mill rate, not Saskatoon's.
A Saskatoon figure applied to a Martensville address will be wrong in either direction, which is exactly why the comparable sales need to come from Martensville itself. Get your hand-reviewed valuation from Joel Dyck.