in my experience, payment processors charge $.30 - $.50 + 2.X% per transaction (for credit cards). No other monthly fees required, though they'll gladly sell you some.

You definitely don't need an engineer dedicated to monetary transaction flows. We use Braintree and haven't had an engineer touch it more than once a year in the past 4 years.

if you want to go engineering free, shopify has subscription plugins that can handle everything for $20-$80/month...

I feel like your comment is from 10+ years ago, back before competition led to payment processors becoming a commodity... has it been a while since you've looked?