Well to be honest I couldn't give you an average maintenance cost for a average year, been meaning to do that for my own records...
But, your 'normal' maintenance costs are
1. A/C Fix
Note this is all relative, but say you are renting a 3 bed/2 bath, 2,000 ad. sq ft., you are looking at a 3-ton AC unit. Probably every 2 years there is a routine problem that will be a $150-$300 repair job. Note, as the unit gets older, the routine repair jobs become more frequent. Long term, probably have to replace a unit every 8-15 years (I just replaced a 17 year old unit 2 months ago)
2. Plumbing
Plumbing jobs are usually just leaky faucets, toilet doesn't flush properly...can't be more than a $200 job.
4. Appliance/Electronic fixture
Fridge, Washer, Dryer, Ceiling fans, honestly I couldn't give you an average cost. You could have everything from an ice maker repair to the duct on the back of the dryer is loose, to just needing a new fridge ($400+).
3. Natural Disaster
Depends on your location. I'm in South Florida, so hurricane season is my main concern. Yours maybe tornadoes/flooding. Roof damage/leaks are public enemy #1 round here, obviously you have your homeowner's insurance to cover costs, but, naturally, going through the claim process is a bore. Also, obviously your tenant would have to vacate the premises if it's a lengthy repair job.
4. Move-out repairs
When you are in between tenants you will 'see' your costs more clearly, lol. You'll see baseboards that need to be repaired, walls that need to be repainted, re-do grout in the bathroom, change locks, you name it. Costs range.
One thing you can see from this list is that if you have some handyman experience, you can cut your maintenance costs handily. I'm 22 and was kinda pushed into the rent business, so the only maintenance I can do is cut the grass while the house is vacant lol.
If you don't have maintenance experience, getting a reliable handyman is your next task. If you have a weekly mail circular (in So. Fla it's called 'The Flyer') you can call around and get quotes, have people come out for an estimate. I have separate people, one guy for A/C, trusty old guy, and one guy for general repairs (plumbing/paint/tile, etc.)
If you are trying to do a spreadsheet of income-costs, remember to do your homeowner's insurance/property tax. Usually you can see if renting will be profitable/breakeven/loss with just those three variables (rent-homeowners-property tax).