Cost of maintenance of a roadbike I am going to buy a bike and ride upto 1000 km or more in a month. Adding them, I think it will come to 12 k to 15 k per year on an average. 
I will be riding on highways, so there won't be any off-roads. Nevertheless, the roads in my country are not well maintained.
Let us consider that initially the bike is comfortable and everything fine at the time of purchase.

What are the different problems (repairs &amp; maintenance) that occur?
Among those which ones will cost me and which ones will be free of cost.
Also if it is possible, answer them with 'high, medium, low' cost of maintenance.
There are a few things that are going to cost you more, simply because you are not used to working on a bike and will have to have them done for you.


Tuneups - You generally get one free at about 30 days after you buy the bike. Mostly this consists of readjusting the derailleurs (shifting mechanisms) and the brakes as the cables stretch. Tuneups in my area (Phoenix, Az) cost between $50-75 depending on shop and any replacement parts. Avg 2-3x per year
Tires/Tubes - Ongoing. I recommend a tougher tire such as gatorskins ($50 each) for training, and then switching to a performance tire (Conti GP 4000 - $65 per tire) for racing. Tubes range from $7 to $15 each. I would just stick with butyl, even latex for racing is a bit overkill to start. Avg 6-8 tubes per year, 1-2 tires (per wheel).
Chain/cassette - $50-100 per cassette, $20-50 per chain depending on quality, plus shop cost to install/change. 1-2x per year depending on maintenance and mileage.


Those are the major ones that I would consider regular expenses. Other expenses such as new rings for the front crank happen, but they are much less often. Same with changing out the seat. Now, you have to start looking at your own personal maintenance habits. Are you going to be cleaning and oiling your chain on a regular basis? If so, then you can add cleaning/degreaser solution, and chain lubricant to your ongoing costs. Poor maintenance/dirty drivetrain will cost you both power and lifespan of your components.

Other incidental costs (Generally less than $10 for item cost, plus possible shop time for install) are brake pads, cables, bar tape, cable housing. 

Now, you can get top of the line components and wheels, just realize that your ancillary costs will go up as well. For example, if you get performance wheels with carbon brake tracks, then your brake pad cost goes up from $10 to in the neighborhood of $50.

Finally, the initial cost for tools and other items to be able to do your own work is high (A complete home type set of park tools can be assembled for $100 on the sparse side to $200 for almost everything needed), the payoff is that you save shop costs forever. It's all a matter of what you personally feel comfortable taking on.