Pricing per square foot is the least accurate way to price stained
glass. Pricing per piece of glass is much more accurate. Here's an
article you might find helpful.

http://www.glasscampus.com/phpBB2/index.php

As you do more work, you'll become more familiar with how long
different elements of work take and then you'll be able to accurately
determine your costs. It's important to differentiate between costs
and prices. Cost is what you paid to make it. Price is what you can
get somebody else to pay your for it. Cost will remain constant, but
price will vary depending on where you sell. What it costs to make is
irrelevant to your customers. They don't care. There's no such thing
as real value - only perceived value. A customer will pay what they
believe it's worth without consideration to your costs. The "trick" is
to figure what they perceive it's worth. It's simple math to cost.
It's an art form to price.

If you're serious about building a business selling your work, I
strongly suggest you ignore suggestions to "not sell cheap" but instead
start by selling as cheaply as you can live with. That's how you start
a business - by creating momentum. Your goal should be to sell enough
work to be full time busy making work for sale. When you're selling
everything you have time to make, you start increasing your prices -
but never so much or so quickly that you're no longer busy full time.