Morning Simon,
Having played with both a CA Vampire and the Alleycat Vampire, there is no doubt in my mind that the Alleycat kit falls together in comparison to the CA offering, and is worth every penny.
To get an idea regarding prices, your best bet would probably be to have a hunt on eBay (which I know can over inflate prices frequently) and see what kind of price these kits tend to go for. Ignoring the more ridiculous sales such as Meteors for the £70-80 mark, you can probably realistically expect to pay £40 or sometimes less for one of these. On the same note I've seen Vampires and Venoms from CA go for ridiculous prices as well (the Sea Venom comes to mind), but you can often get them for around the £20-30 mark.
They are a lot of work as being limited run there are some fairly hefty fit issues with some of them, but for a reasonably experienced modeller, they shouldn't be too difficult to get decent results with.
Nik