You're overestimating the cost of bulk VPN services. It's like $1.50 per click worker desktop PC terminal per month, and that's if they don't have a decently skilled sysadmin to DIY it.
If you limit the IP to the residential IP space then VPN costs become huge. You need some resident to rent his IP to you. If those residents get punished by their providers for that behaviour then soon you will have to rent apartments to buy your own connections. That's hundreds of dollars.
Now, if Amazon would be sincere about preventing fraud, they would ban the IPs of fraudulent comments and somehow cooperate with the providers if they use random IPs. In other words, you would have to rent a new apartment for every bad review. That's an exhibitive cost.
I didn't make any specific cost estimate, merely stating "some". I'll grant that $1.50 is actually "none" iff it's a negligible fraction of that worker's monthly earnings.
With "wages low skilled" at 4650BDT [1] that's $55.50 at today's exchange rate. I'm standing by my estimate of "some", as a 2.7% "tax" on income isn't negligible, and could have far greater significance for those who earn at the bottom of the scale.
