Screens are priced per square foot.  When measuring round up to nearest whole or half foot.

Pricing per Square Foot:

Standard screens
$3.65 for Black or Brown Suntex 80% 
$3.80 for Black or Brown Suntex 90%

I do not stock the lighter colors due to lack of demand, I can special order them at a slightly higher cost. (see the reason here)

Minimums
The minimum screen size for pricing is 4 square feet.
The shortest dimension used in calculating is 1.5 feet.
(A 4' by 1' bathroom window becomes 4' by 1.5' or 6 sqft.)

Other charges  (Sales tax is also added on all orders.)
Add $10 per screen for unsupported second floor or vaulted installation*
Arched or custom windows (including grid work) will be priced individually,
  call or e-mail for a quote.
An extra $35 service charge will be added to orders totalling $350 or less.

*If the top of the window is more than 10 feet off the ground it falls into this category.

Where possible I like to keep the maximum width of a screen less than 48 inches.  This helps prevent the screen from sagging and provides a nice look outlining each section of glass.  Most windows that open horizontally will take two or three screens, one for each section of the window.  Smaller screens are also much easier to handle when removing to clean the window or store in winter months.