The more people that sign up, the less likely it is that you'll win. Furthermore, if you have to pay extra for each transaction, it can get very expensive to stay competitive. You may earn a prize at the end of the season, but still lose money depending on how many transactions you had to make. League prizes, on the other hand, give you the same chances of winning regardless of how many people sign up. You must decide whether you want to compete for a large grand prize, which gets more difficult to win as more people sign up, or whether you prefer a smaller prize with a much better chance of winning.
Some services have both, but they generally have much lower league prizes.
They would need 2000 people just to pay the prizes. They still have to pay their advertising costs, overhead, salaries, phone bills, mailing costs, etc., before they make any profit for themselves. Ask if the prize money is already available and guaranteed. Then ask when the prizes are actually paid. Some services count on income from football entries to pay out their current baseball prizes.