Mew* is worth quite a lot with collectors (you can get several Exs in the range good to very good for it), but a pure player such as myself won't give a lot for it.
A true priceguide doesn't really exist. After all, prices are determined by playability (which differs per region, a Metagross Delta is worth more in the USA as in Europe) and Rarity (Tho Holon's Castform is worth more as some Ex's). So to each, certain cards have other value. I for example value Gardevoir PK higher as most people because I need it.
But your Mew, I'd say you should trade it for some Exes with a collector.