The amount of money that you spend on balls can vary greatly.  Growing up, I almost never bought balls and just used the best that I could find.  For a lot of you, that means stealing 3 or 4 out of our teaching baskets.  The coaches add our used balls to them all the time, so feel free.

If you are going to buy them, here is some information:

Standard Balls (about $2.50/can).  This is what we use for high school matches.  These are usually Penn or Wilson and play like new for about 60-90 minutes after you open them.  Their bounce will decrease by about 20% pretty quickly.  I heavily prefer Penn to Wilson - I think they are much better.  You can frequently find cases of 20 cans of Penn balls at Costco for about $40 (sometimes on sale for $30 - let others know when you stumble onto this).

Premium Balls (about $4 if you buy them by the case (24 cans).  These bounce a little higher to begin with, will play pretty well for about 3 hours and lose a little less of their bounce over time.  These would be Pro Penn, Wilson US Open, Babolat Gold, Yonex (can't remember the name) Dunlop ATP.  Pro Penn was the best for a long time, but their quality control has been terrible lately.  So, I'm using the Wilson US Open currently.  If you want to play a couple matches with the same can a few days apart, the Dunlop is a good choice.  They are a little hard when you first open them, but hold their bounce over time pretty well.

The Best/Worst ball on the market

(price in flux, about $100 for a case of 24 cans):  Wilson Trinity - the first eco-friendly tennis ball.  This ball is designed completely differently from other balls, but still plays fairly similarly.  The great thing is that they only go flat by playing, versus other balls that start to go flat once the can is opened.  These are a great option if you want to use balls for 2-4 matches and want them to play like a new ball for all of them.

These are actually best (honestly, by far the best) for keeping in a basket for drills, serving practice. If you keep them in a room that doesn't get freezing cold or super-hot, a basket will play like new for a very long time (like 1000 shots per ball).  On the down side, they tend to be a little bit hard and make a slightly different sound when hit.  So, a lot of the people I play with refuse to use them.

Why play with new versus used balls?  As noted, balls will lose a bit of their bounce over time.  In general, that makes them a little easier to keep in play.  Thats great, but every competitive match that you play (team matches, tournaments) will be played with new balls.  So, switching from old to new balls (that are harder to hit in) for the matches puts you at a disadvantage.  For the same reason, I'd also play with the standard balls instead of the premium ones (thats what 95% of your high school matches will use).

Why play with used balls versus new balls?  You will save money.

So, balance those two for yourself.

Tennis ball prices can vary by store.  For a long time, buying online was way more expensive than going to Costco (low-end) or Players (premium balls).  That's not necessarily true these days.  Tennis Warehouse, Midwest Sports and Do It Tennis are places to look for balls online.  Amazon also has them, but their prices CAN sometimes be stupidly high.