The first question is (in my humble opinion) not good when comparing to industry. I look at what the others in my area have, what they are offering, and how old they are. So, mine is more based upon the city/state you are in. I am sure depending on the city/state you will see a range from 15 - 50%. Mine is when I see it above 35% to start looking at what the reasons are. Out going surveys by and independent source is good to have as well. Can give you some insight, not saying they all with be truthful and upfront, but I look for common themes.
Turnovers can cost you from $1000 - $4000 depending on how long the tenants were there, what checks you were doing in units while they were there, age of property and appliances, what kind of purchasing power you have.
When planning budget for move outs I look at historical data and averages and I look at the 5 year trend. Then I look at all leases ending by month and I make an assumption that they are all moving and that is just me, because I know I will not come up short. I am of the mind rather over estimate what I need rather than under estimate.
Retention incentives, I put them as having an exceed expectation maintenance team, that is very proactive in fixing maintenance problems. If, It cannot be as an example appliance fixed in 24 hours, then replace it with another appliance and fix the appliance in shop. If it is a part and it cannot be obtained in less than 24 hours, replace and fix in shop. It is aggressive and there are those who will argue it costs too much money and in the long run this arguement has never been true from my end. Then besides maintenance having great customer service from the manager on down. Being attentive to the tenants and spoiling them and getting rid of those that take away from community being safe and a good place to live. So, gangs, drugs, prostitution, and unsupervised minors are no, no's. So, my incentive is the service you give and actually it is a no brainer with the customer because they want to stay. They know when you care and take care of them.