You should play with whatever your comfortable with. A unit for you or anybody can be $10-1000. That's all relative. Play NO MORE than 5% of your bankroll on your highest rated picks. Stay away from services that want you to play 10,20,50 units, you'll stay in the game longer. The mutual fund is an OK analogy in that your sevice or fund manager has a track record and you can guage what their yearly return will be. Keep your expectations realistic.
With math stuff and percentages I think Above the vig has most, if not all the answers.
Hope this helps.
vytas