Well it’s unlikely you’ll get the tickets at face value from the official sources as they sell out too quickly. So you’ve 3 options
1) Buy over the Net in advance.
2) Buy it off a tourist agency by shopping around when you get there. I recall one of the agencies behind the Copacabana Palace gave me the best deal a couple of years ago.
3) Buy it off someone outside the Sambódromo. This approach worked for me one year as I got a good deal off fellow salgueirenses and paid 30% less than my Brazilian friends had paid for the same Sector.

I would advise you to wait till you get there and do no. 2).

If you just want a taste of it and are keen to save $$$ you have two further options
a) Buy a ticket for one of the cheaper Sectors (poor view) e.g. 6 or 13 which can be had for $10. Bear in mind that the better Sectors 3, 5, 7, 9 may be $100-150 face value.
b) Attend the “Parade of Champions” the following Saturday.
Adv : Tickets cost half as much and you see the best 7 schools.
Disadv : It doesn’t have the competitive edge of Carnaval.

The last couple of years I’ve seen a report in O Dia on the prices when they go on sale in December which will give an indication as to this year’s face values. I’ve 10-15 friends going to Carnaval this year so will need to find out more anyway.

As for the balls it depends whether you taking about the informal ones at Scala in Leblon or more formal affairs.