Can one assign a performance score to a stored proc to compare performance of 2 procs
When you review/compare a SQL Server Execution Plan, each plan is given a cost value. Although these values can indeed be used for a relative comparison between plans, it is often more beneficial to dig a little deeper than that.

For example, you could compare the execution plans of the two stored procedures, perhaps using a tool such as the Free Plan Explorer from SQL Sentry.

This would allow you to break down the "score" you seek into more useful metrics, such as execution time, CPU consumption, Disk IO etc. and use these for your comparison.

Using this method would enable you to establish exactly where your performance gain(or loss) is achieved.