It would help to know what stadium you're asking about.
Generally, the seats near home plate will give the best views. They also tend to be expensive, and at the ballpark I go to, would exceed the $100 budget for two tickets.
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Took a look at the seating/pricing diagram for Nationals Park. For a budget of $50 per ticket only, I'd probably opt for the left field baseline reserved, or for a couple dollars more, the left field baseline box seats. I much prefer to be down in the lower bowl than in the upper deck, and I prefer third base side to first base side, as you get better views across the field to plays at first base.
If you want to be more behind the plate, you could opt for an upper deck seat.
This tool also shows the field view from a particular section. http://washington.nationals.mlb.com/was/ticketing/seats_3d.jsp
Note, though, I've never been there, but given that it's a new park, I find it hard to imagine that there are seats that are actually bad.