In Yahtzee with Buddies or Dice with Buddies, tourney is one of the best way to earn some bonus rolls. However, with the current setup of tournaments, they are really pay to win in disguise. However, with better strategies you will have a better shot at turning a profit in terms of bonus roll spent and bonus roll earned.

First you will have to estimate the number of Yahtzees required to reach the winning point requirement. Please note that this is just a rough estimation.

2 Yahtzee: 300~400 points.
3 Yahtzee: 400~500 points.
4 Yahtzee: 500~600 points.
5 Yahtzee: 600~700 points.
6 Yahtzee: 700~800 points.

Each Yahtzee will require roughly 5~10 Bonus rolls to achieve, or even more if you are unlucky. This means that to win a tournament with roughly 500 points to boot will require you to have at least a bank of 40 bonus rolls (10 roll x 4 Yahtzee).

Now, estimate the amount of bonus roll awarded versus your potential investment. If the tourney awards 75 bonus rolls to reach a score of 500x, you may be able to turn a profit by reach that top slot. However, someone else may try to overtake your top award and you will end up losing if you only receive 20 bonus rolls. Your odds and lost will get even worse if you pay bonus rolls to get into the tournaments.

However, you should always take advantage of the daily free entries to have a shot at getting that top 3000 slots.

Tournament Scoring Strategy
Since your goal in tournament is really to beat the odds at getting higher scores. You need to make some adjustments to your overall strategy. The majority of the higher scoring games are achieved via multiple Yahtzee, thus you will need to focus mainly on x-of-kinds roll while shooting for that high score.

Always hold those pairs and three of kinds in favor of going after straights, break your full houses if you still have the rolls. Only record large straights if you get them right off the bat with your first roll. The rest of the game, focus on the upper section with a heavy focus on possibility of getting Yahtzee.

In short, the optimal strategy against other players does not work in a tournament setting.