Seven years ago I accepted an employment offer from a friend who had
started a web company that was thriving and he needed someone to run
the company so he could go overseas.  I accepted the position knowing
that his intention was to eventually sell the company and I was told
that I would be given a sizable bonus if I committed to staying until
the company was sold.  Over the past seven years we hired 3 more
staff, introduced 3 more revenue streams and built the site into one
of the top 5 sites in our market.  As we hired new staff my role was
reduced to mainly advertising sales and finances but as with any small
company, I had input in all areas of our operation.

Over the years I have been offered other positions with different
companies but I did not persue them because I had made a commitment to
stay until our company was sold.  Now that time has come and the
company has been sold.  My question is, what percentage of the sale
price is it fair for me to expect to receive as a severance bonus?

Let the value of the company when you joined it be x.
Let the value of the company when sold be y.
Then the company increased in value by (y - x).
Let z be the fraction of the value increase (y - x) that you personally created.
Then it is fair for you to expect z(y - x).
You asked for a percentage of the sale price y.  That would be ((z(y -
x)) / y) as a fraction, or ((100z(y - x)) / y) as a percentage.
Unfortunately, a reader of this page cannot tell you what z is, and
even observers close to the company might well disagree on its value.

I think 10% would be fair. Anything  more would be a pleasant
surprise. Anything less would be fair since I don't know the relative
size of "sizable bonus".