I too receive free tickets every free roll, but since the price is only 1 Satoshi, sometimes I'm tempted to buy more myself, especially when I see that people frequently win good prizes with around 1k tickets(an amount you can't accumulate from free rolls in one week.) As for 1 ticket for every 0.00000500 BTC wagered, I decided that I'd rather buy 1 ticket for 1 satoshi, but if you play Multiply BTC game anyway, then it's a good bonus for you.
It doesn't matter how much a ticket costs. It doesn't make any difference even if each ticket costs 1 mBTC. Because you are not the only person who buys tickets at 1 satoshi.
Also, your chance will not increase if you buy tickets at 0.1 satoshi.
Assume that 200 million tickets have bee distributed among participants and each ticket costs 1 satoshi. You spend 1 mBTC and buy 100,000 tickets. In this case your chance to win the first place of the lottery would be 100,000/200,000,000=0.05%.
If the price of each ticket is 1 mBTC, your chance of winning doesn't change. In this case you will buy 1 ticket with your 1 mBTC and total number of tickets will be 2000. Your chance will be 1/2000=0.05%.
So, if you think buying lottery tickets is a good opportunity because they are cheap, you are completely wrong.
It's like that someone says I prefer dogecoin to bitcoin, because a dogecoin costs only 0.002 dollar. Instead of buying one bitcoin I can buy 4 million dogecoin. Total supply should be also considered. And in a lottery you should pay attention to total number of tickets and also the percentage of the total pool which will be awarded to winners.