> How much is the promo Mew card from the TCG League worth?

It is too early to tell.  We collect data for prices from internet
auction sites, internet singles dealers, and random local singles
dealers.  We just don't have enough information to put something
up.  I'm sure it will be in soon however.  Keep watching PoJo's
price guides page for updates.

This seems to be a good opportunity to explain the price guides.
They aren't perfect.  They can only reflect the data that goes
into them.  If a magazine primarily collects data from store front
shop owners, then prices will be different from a source that 
primarily collects data from internet auction sites.

PoJo.com tries to balance all of these interests.

However, in the end, a card is only as valuable as someone will 
pay you for it.  Although most card shops may stock singles at higher
prices than auction sites, they rarely will pay more than 50% of 
their selling price for a customer's card.  They have lots of 
real world expenses to cover, and this is where they make it up.
And many times they just won't be interested in buying cards.

And PoJo.com does NOT buy cards ;-)

Use the price guides as a rough measure of what your collection
MIGHT be worth if you spent the long, hard effort of finding buyers
for the cards of interest.  Or better yet, use it as a guide to completing
trades among Pokemon collectors.  Relative values will help you ensure
that your collection does not diminish in value from trading.

You should certainly not RELY on converting your collection to cash
based on PoJo's (or anybody else's) price guides.

I hope this helps,
Brian Brokaw