I recommend all my students to get a good calculator. Picking the right calculator is very important. A good calculator can save valuable time on standardized tests and exams, so it is important that the student familiarizes with a good scientific calculator early. Today there are many excellent choices but also many poor ones. Not only does the readily available responses and easily enterable input make experimenting with different formulas fun and homework less stressful, but also it is a useful tool in solving complex problems, a learning tool, and a motivational tool.
There are several things to consider: the level of math, price, and preference. The best calculators that I have used and encountered are the following:
Scientific Calculators (for grades 6-12). $10 - $20
1. Casio FX-115ES PLUS
2. TI-36X Pro by Texas Instruments
Graphing Calculators (grades 9-12). $100 - $150