In the first week of his freshman year, my kid's high school math instructor presented him with one of two graphing calculator suggestions.  The choices were the Texas Instruments TI-Nspire CX and the Texas Instruments TI-Nspire CX CAS.  For a detailed breakdown, visit Texas Instruments' webpage on exam acceptability.  

ti calculators approved for ap ib act sat

I presume that the CX is supposed to be the more expensive model, but street prices for either of the two models is about the same, somewhere between $125 and $160 dollars.  Texas Instruments even has a webpage that gives retail price comparisons.

As for other brands of calculator, they are accepted.  Be sure to check out each test's webpage for details.  In general, (excerpted from the PSAT/NMSQT site)

Bring One of These Calculators

  • Graphing calculator (most models permitted; see Official Student Guide for list)
  • Scientific calculator
  • Four-function calculator (not recommended)

Leave These Devices at Home

  • Anything that can connect to the Internet
  • Anything that makes noise or “talks”
  • Calculators that use electrical outlets
  • Calculators with QWERTY keypads
  • Tablets or cellphones
  • Laptops or handheld computers
  • Paper-tape calculators
  • Calculators with a pen-input device or stylus

Because both the TI Nspire CX and CAS appear to be well-supported, and because my son's math teacher is familiar with either of the two, we went with the CX.  That way he can use it with any of the upcoming tests he may be taking.