It's very easy to calculate reward probability with a little information. What is the Network's Global hashrate, Blocks/day, and your percentage of the global hashrate. Because of the law of averages and luck involved in hashing algorithms, your variance in hitting a block time-wise will average out over the higher amount of set data.
Simply divide your hash power by the global, multiply that by the average blocks per day times block reward (e.g. Ether 5, Zcash 12.5) to find your maximum pool reward per day. Ether is supposed to average 20 secs per block or 4320 blocks (21,600Ether) per day, but this will have variance. at 20MH to 100TH, you represent 0.0000002 (.00002%). So if you joined a pool earning 100% of the blocks you would make .00432 ($1.30@$300/ether) Ether per day, assuming accurate share reporting. Pools vary at block earning ability so do the exact same calculation using the pools stats instead of the network and you can estimate your earnings per day.
As for solo-mining, use a similar line of thinking. If you expect .0000002 out of 4320 blocks per day that is .000864 blocks per day it would take 1157 days before you should expect to have mined 1 block (coinwarz at 20MH calculated 1341 days.)
That being said, you should keep in mind that every block that gets rewarded is pretty random. The pools are filled with ppl runing all range of hashrates. Think of it this way, imagine a giant pile of lottery tickets and everyone one in the network is furiously digging through the pile to find the matching ticket. It's guaranteed there will be 1 or 2 winning tickets per round. Now, what are ppl using to hash with? 1 Gpu, multiple Gpus, or multiple systems etc. Say someone is hashing 100MH with 5 GPUs identical to yours in 1 system. Each GPU is getting the job from the miner and attempting to find the winning ticket. This person can throw away 5 tickets every time you throw away 1, so they are 5 times more likely to find the right ticket. But each ticket has the sames odds! Meaning they have 5 equal tickets to your 1, not 1 ticket with a 5 times higher chance of winning. Does the distinction make sense?
Anyway, you might consider looking into new and upcoming coins. The dollar per day earnings can be higher with a cheap currency. You could pool or solo mine many coins with lower market caps and come out better than with Ether. And it is easy to trade through the exchanges like bittrex.
Hope this helps.