In many places, the available public right-of-way along Long Point for the street, sidewalks, trees, and utilities is approximately 60-feet wide. Therefore, the acquisition of additional land to create an 80-foot right-of-way would result in a significant price difference versus a 70-foot right-of-way.
Exclusive of ROW costs, when designing a 70-foot versus 80-foot street, it would depend on what improvements are desired (medians, sidewalks, better transit stops, trees, etc.). In general, an 80-foot street requires more work than a 70-foot street. But depending on design decisions, an 80-foot street will not necessarily cost more.