From the viewpoint of enlarging a city’s effective labour market (1)/and economic output, they are therefore (2)/ very important to keep the commute time short (3)/and commuting cost cheaper (4)/within a city as they keep growing in population (5).

2- here, ‘they are’ is incorrect. From the viewpoint, ‘it is important’ is correct. 4- ‘cheaper’ is incorrect here. The first adjective is in the positive degree. So, ‘cheap’ is correct. No need to use comparative degree. 5- we are talking about a city. So, ‘they’ should be replaced with ‘it’ ad ‘keep’ with ‘keeps’. From the viewpoint of enlarging a city’s effective labour market and economic output, it is therefore very important to keep the commute time short and commuting cost cheap within a city as it keeps growing in population.