I'd put emphasis on the "can be", it very much depends on your read/write patterns and configuration. Assuming you know those up front and they fit dynamo well it can be several times cheaper than any sort of SQL.

Every time I've made use of it it's ended up costing pennies compared to what SQL would cost, sometimes literal pennies. If you turn on all the fancy features from day 1 and fill it with tons of data you don't need and make too many reads/writes per-request though you can get into very pricey territory very quickly.

We tried to aim for 1 read and/or 1 write per request to our service and that worked really well for our use cases. It kept costs low and performance high but we had a really well understood problem. If I was a startup and didn't know quite how my product would turn out, I don't think I'd consider dynamo for a while.