This is an incredible tool, one that is currently the most popular, and the most widely used in the UK for valuing CGC graded comics. GPA is an information crawler that collects recent sales prices from all over the internet, then compiles the data to give you a selection of different valuations, from 90 day averages, to sales from the past three years.

The only downside to this is the fact that it is an American crawler, and often it appears that it is limited to crawling American websites, so the values in the UK can be off.  For example, UK price variants on GPA tend to be far lower than their actual value.

Pros

- Easy to use

- Informative and comprehensive

-Likely one of the most accurate valuation tool out there

Cons

- Requires a membership fee

- American based, values sometimes do you reflect the UK market

Check them out here