Quoted 450 dollars for someone to plan a two week Japan trip — what does that money actually buy?

There are two different products sold under the same word and the quote you have is the weaker one. A travel agent books things, holds the relationship with hotels and operators, and is who you ring when a flight cancels at 11pm — often paid partly by commission. An itinerary planner sells you a document, which is worth exactly as much as the local knowledge inside it. For a first Japan trip the document version can genuinely earn its fee if the planner knows the country, but ask what happens when something goes wrong on the ground, because usually the answer is nothing.

I paid a similar amount for a Japan itinerary and the specific thing that made it worth it was the rail maths. Whether a rail pass beats individual tickets depends entirely on your route and it changed enough in recent years that half the blog posts you will find are simply wrong. The planner ran our actual route both ways and the answer was the opposite of what I had assumed, which covered the fee on its own. Everything else in the document I could have found myself.

Cheaper alternative that gave me more: hire a local guide for one full day at the start of the trip instead of buying a two week plan. Half a day into it you understand how the transport works, how to order, what things cost, and the rest of the trip stops being intimidating. It cost less than that quote and it changed the whole fortnight rather than just the paperwork.

I paid for a planned itinerary once and regretted it, because what came back was six pages of the same headline sights I had already shortlisted, scheduled tightly enough that we abandoned it on day three. The failure was that they never asked what pace we like or what we hate. If you do go ahead, judge them entirely on the questions they ask you before they write anything.