Priority should be given to channels believed to meet the communications objectives at the lowest cost. However, because of diminishing returns, the spending on any channel should be kept below the level at which it stops being a good investment.
In the typical scenario illustrated here, a brand should use the low reach channel if the budget is less than $X and the high reach channel if the spend is more than $X. The most efficient option, however, is a mix of the two channels, especially if their audiences do not overlap much.
This short video explains the concept in more detail.
For further insights about media planning, check out this short article or listen to this audio clip.