To have one made in flash from scratch and if it was all to be intergrated into the website, you would be looking at least £1000 + for that.
IMO you could count it based on these breakdowns.. The website, the design, the hosting, the games modules, the payment gateway the website : a custom made cms arround $500 the design, quick browse through a template provider should gives you a nice design at arround $100 the hosting -- this is recurring and the prices is fluctuate, specially you need a good offshore hosting account the game-modules, flash based game namely bluejack 1 module of this kind of flash game requires arround $75 ~ $150 (depends on graphics, animations, and sounds) and the last but not least you need a payment processor provider
I think the biggest cost factor would be whether it's single or multi-player. Single player = maybe under $1 000 Multiplayer = More they $5 000