Martensville's own assessment and mill rate, and its own appeal process rather than Saskatoon's Board of Revision, are what actually set the carrying cost on a two-family there, so a Saskatoon-based cost comparison can be misleading without checking Martensville's numbers specifically.
Whether a legal secondary suite there can be financed still depends on permits from the municipality itself, and the Residential Tenancies Act, 2006 governs the lease regardless of which municipality the property sits in.
A two-family in Martensville can absolutely work as an investment, but it should be underwritten on Martensville's own numbers, not borrowed Saskatoon figures. Get your hand-reviewed valuation from Joel Dyck.