Ughh how confusing and not nice that is to take a whole %30 of each copy sold!
30% is common for most digital retailers, but that covers hosting and distribution, as well as processing payments and handling refunds. You could do all that yourself with your own website, but it takes time and money upfront to get that sorted.
There are other things to consider as well that affect how much you'll get paid.
In some countries (Turkey, Russia, China, etc) the price is 50-70% less than the USD price, then there is sales tax/GST/VAT that might apply and could be up to 20%
So, worst case scenario all purchases of your game are in the cheapest country and sales tax applies, before Steam even takes their share
((($5 - 70%) - 20%) - 30%) = $0.84
That would be around ~120 copies to get paid $100
That's not even factoring discounts, and most of your sales will come when the game is discounted.
Then there might be intermediary bank fees, which could be something like $20
And if you're not from the US, then there could be fees for recieving foreign currency, e.g. $10
So now you want Steam to send at least $130 so you get the equivalent of $100 after bank fees, meaning you need to sell ~155 copies
Back in February 2018, the average sales per new indie game was ~50 and it's trending downwards
gdcvault.com/play/1024976/Let-s-Be-Realistic-A