With this payment, Nice Hash pool will give you this power for a few days at today's price
Now, according to the calculator book, with this power, at least 61 days of blocks can be obtained.
How many days did you manage to gain power with this money?
I think it will be 30 days, right? If I am right, your chances are 50/50 and this is relatively good.
In your first sentence you (correctly) state that they can buy a few days worth and in your last sentence you state 30 days, does not compute
So I don't know how you got 30 days but the group run uses about 0,18 btc and with that you can buy approximately 1 day and 17 hours (~1.7 days) worth of 25PH so the odds are something like 2.8% of hitting a block (1,7 / 61 * 100) or very roughly 1 in 36.
By the way, easier way to roughly 'calculate' this is to just divide the block reward (6.25) by the used amount (0.18) to buy hashpower: 6.25/0.18=34.7 The odds in reality are a little worse because of Nicehash fees and the fact that hashpower is more expensive there.