normal times was well within the free tier, and this was even with a pre-warm job turned on for our endpoints. Even the spikes were not very expensive. Back then 80% of our bill was RDS and I think we were paying under $50/mo for beyond-free-tier usage of lambda. It was a tiny fraction of what the cost was when we had EC2 clusters.

If you're ok with cold-starts on less-used routes you could probably make any medium to low traffic app run totally free on jets (other than db)