Not much, 50-100$ worth bitcoins just to start with it and to see how it works.
But here is problem - if they buy bitcoins for 100$ they will get that minus 25-30$ fee and they will pay fee for next transaction, so their 100$ worth bitcoins will become 40$ worth bitcoins. How to explain them this??
Is this really what bitcoin become? Currency used for miners so they can earn more money?
Well, today a fee of 400 sats/byte is going to get you confirmed pretty quickly, whereas three days ago it would have taken more than twice that. It's unfortunate that of the past 12 months they decided to enter right now when the mempool is full up (but hooray it's coming down slowly). Though right now I wouldn't even mind waiting a few hours more (6 hours estimate according to bitcoinfees.earn.com) for 190 sats/byte. For an average transaction that should cost around 50k sats - 7 dollars. So you're getting ripped off by the fees of the exchange you're using.
Why not use localbitcoins? What you pay for is exactly what you get. Your maths is way off, too. 100-30 is 70, not 40. This isn't what "Bitcoin has become". You've always had to pay fees at exchanges. Try buying an alt, same spread, same fee.