No quick answer to this question.
A lot is going to depend upon your level of technical knowledge and ability, the time you are willing to put into this and your expectations and the amount of money you are willing to carry as a monthly cost.
A low cost non-database site can run for under ten dollars a month, we are currently spending about 35 a month for a database-based site running with SSL for login.
It is fairly easy to set up a site. But after that it starts to become hazy.
1- How big is your area ?
If you are in a small town with no major cities right next store, then SEO to get listed on the first or second page of search engines is fairly straight forward and pay-click like adwords from google could be viable. You might even make a few bucks a month on advertsing using a service like google or CJ to help offset the costs.
If you are in a major city, forget about it. You will be lost in the hundreds of other sites and the costs of adwords will quickly become prohibitive.
2- How interested are you in leaning SEO to help get your ads in decent positions in the search engines?
There are literally tens of thousands of rental websites out there on the net and a lot of them let you list for free. Most offer some type of syndication ability, copying your ad to other sites, often creating a ripple effect across the net.
For example, list your unit on Oodle and it will appear on Facebook and dozens of other sites that use Oodle as a consolidator, my husband and I do this for our own site.
Given the continuing non-justifiable costs these days of the paper adverts, it is wise to add the internet to your advertising for tenants, the costs and viability of your approach depend on some of the factors listed above.
GL