It should be $100. If it is coming and going then that's probably just because the exchange rate is changing.
I just checked and I see that I have to deposit ~0.01 BTC to play without captcha, what is around ~$60. So, I suppose these deposit requirements are different for every account.
It reduces when you wager or buy lottery tickets but $100 balance on its own will be enough for people that don't wager.
We've added a new feature for all you captcha haters. You can now use Reward Points to avoid captchas. It starts at a cost of 1 RP and increases by 1 RP when you use it. The cost will come down when you don't use it with a cooldown timer based on the ratio of RP rolls vs captcha rolls.