It was almost entirely based on answering the phone when it rang, offering good customer service, keeping the place cleaner, and actually selling units when people came in to rent them. 

The drain cleaner snakes or jets the line and rarely are they able to see the items flowing past.Also in some cases it's a combination, it could be an old cast iron pipe with a rough inside surface full of rust, which started to act like velcro to things that flows past, or a slight settlement causing a belly, which again impedes the flow, then the tenant pours down grease, pasta and bean, well, you get the idea.Most of the time you end up eating the cost especially if the plumber can't tell you anything.Now, you mentioned that the clog caused water to back up to a vacant unit next door. 

I guarantee that place is cleaner than you think. 

Wow, my cleaner takes care of the grill when she flips the house.

Not every buyer will want one but some with allergies or needing cleaner air it will be a selling feature.

There are tenants with cleaner backgrounds and fewer questions than this.

The Landlord should send a professional cleaner through and clean behind the stove and in all the cabinets.

If you just would enjoy having someone make your food, I don't see a problem with that as long as it's worth the money trade off.we have a house cleaner and have for 20 plus years.. 

Aaron Brooks over at Quad City Cleaners has been my go to lately. 

Then, clean the place up and put an ad on zillow for $1700 per month and you might get lucky and get a tenant who is willing to pay $200 per month more than the market that you only think is the maximum for the area because some fool wants to live in your rental unit because it looks cleaner.$1700 - $975 = $715 x 120 months = $87,000 more for rents in 10 years and that does not include annual rent increases that look like the following chart.