Ok, I quickly did the math for 500 lambdas (1Gb):
Provisioned:
500 * 86400 (sec/day) * 0.000004167 ~= $180
Keep-alive:
500 * 1440 (min/day) * 2 (sec. runtime) * 0.000016667 (100ms price) * 10 ~= $240
Invocation costs on the provisioned ones would be a lot cheaper too cheaper too.
Roughly $45 provisioned vs $80 for 50M calls.
So depending on the demands, provisioning can be more performant, and cheaper at the same time.