Flash is very useful just for 3 occasions in e-commerce:
* To animate Menus (some)
* To cloak links (to improve PR)
* To create some kind of creative environment (at check out process for example)
Just small bits & pieces! I laugh when I see all those websites with 10 sec. flash intros
It's the stupidest thing anyone could do!
Regarding your question: I think 50k GBP would be enough to get fully stocked video game online shop up and running.