I think it all depends on your trading psychology, if you copy pest another persons strategy it might fail for you. The amount that I am willing to lose is a factor of my risk appetite. Personally I can't risk more than 2% of my entire portfolio, and my risk to reward ratio I set it at minimum...
it all depends on what you are getting out of data, and the vendor but from what the others have said you can basically make a rough guesstimate on the price range. Some vendors are more than willing to offer you discount and from them you can get unbeatable price than you might not be able to...