I wouldn't actually, not for that price, there are cheaper ways to invest in the online gambling market. Also it is not the best Casinos / Bookmakers who actually make the most money/are worth the most.
Look at 32Red vs 888? Which is better company/would you rather have an account with?
Which is valued on the LSE at £12m and which at £360m?
Betfair might be great for many things but I dont think it will make good long term profits for shareholders, not at that price anyway. Everything has a price and that is about 3x too much.