The largest amount you can win via free roll is always $200 and each row above it in the table is 10 times lower. This means that the lowest amount you win with free rolls (i.e. the one you most commonly win) is 0.2 cents in bitcoins. The value of Bitcoin fluctuates a lot even in a matter of minutes and that is why the values in BTC you can win also change. Today 0.2 cents may be 50 satoshis, tomorrow if Bitcoin's value rises 0.2 cents may be 40 satoshis and so on.

Regarding the captcha, as far as I know you should deposit an amount equal to your free roll earnings and it will be removed from your account. You can see your earnings by clicking Stats -> Personal Stats. For example if you have earned 100 000 satoshis, the captcha will be removed if you deposit 100 000 satoshis, but I recommend you deposit a bit more because the captcha may appear again if you continue to earn using free rolls. Please correct me if I'm wrong.

If the owners of the website are interested, here's an alternative captcha that if tuned properly will be beneficial for both users and website owners - it will be easier for the users as they won't have to solve a captcha and will help the owners generate some Monero through mining: Coinhive Captcha.