Texas Instruments, Hewlett Packard, Casio and Sharp all sell basic scientific calculators at reasonable prices. All are available for purchase from Amazon.com, Staples and Best Buy, as of 2015.
Texas Instruments TI-30X IIS scientific calculator comes in various colors and is priced below $20. For another option at that price point, Casio FX-115ES PLUS has similar features, as does Sharp's EL520XBWH. The Texas Instruments TI-30Xa retails for about $10, as does the Casio FX-260 Solar Scientific.
For a calculator with more pre-programmed functions, Hewlett Packard's Scientific Calculator has a two-line display and raised edges. The display on the calculator has adjustable contrast and has edit, undo and delete features. It also retails around $60.