I have rewritten several things from eg. Python to Rust, Rust to Go very recently. The 400 USD is pretty arbitrary as you could rewrite several big projects pretty comfortably with the Claude ~100 USD subscription tier.

In my experience it mostly comes down to the harness (or lack of) that you use. Something like 'superpowers' can be pretty verbose and hash out things for a long time (and use a decent amount of tokens) but the output is pretty decent. The better instructions and the more brainstorming you do initially the better - as the subagents encounter fewer issues.

Without a harness you could try a direct port (prompt: 'convert x to y, don't bother me') and if the languages are roughly compatible you could get a seemingly working port much quicker but likely with major hidden issues. A comprehensive testsuite is obviously a must.