Just because one method costs more does not mean it is more effective. You can post a free ad for your rental online and have a tenant the next day or you could enlist a Realtor to find a tenant and wait three months to fill the vacancy.
You can choose to advertise using only one method or you can choose to use a variety of methods to promote your property. The faster you can find a quality tenant, the faster you will be seeing that monthly rent check. As a point of caution, the focus should be on finding a quality tenant for your unit. If you do not conduct the proper tenant screening and simply put the first person who is willing to pay in the unit, you could be filing an eviction for that tenant the next month and be back to square one.