Is there a reason for this?
Yes, it's very straightforward.
In the "old days" you'd buy one "combined" ticket from one source. These days it's quite common that sites like Kayak, rather ingeniously "put together" various tickets from various sources.
(I recently had to urgently fly to London from the Americas, and saved - I shit you not - three thousand five hundred dollars by doing this. That is not a joke, like well over half a case of Margaux - ! :) )
As you have discovered, the precise downside of such tickets is that:
the conditions vary drastically on the various legs. They are not harmonized.
In the old days, when you about a "combined" ticket, they would be harmonized. So even if your say "third" leg was on Airline X which normally only allows 6kg carryon, Airline X would make an exception (knowing they were part of a "combined" ticket sale) and they'd offer the 12kg that the other airlines in the combined ticket offer.
Similarly you might get free meals and booze on one leg, have to pay for tapwater on the next, etc. You may be able to specify and/or buy seat positions on one leg, but have no option to do so on another leg. You may have convenient online checkin on one leg, but not on another. Etc.
This is indeed the specific, dramatic downside of these "put together" ticket purchases. Like, if you were to ask "So what's the downside of these very cheap fares that are glued together?" the answer is, indeed as you have found out "all the baggage / seat / etc conditions are randomly different throughout."
There is no way around it.
"does anyone know a way to work a rough price out?"
You'd have to just state the airline and leg, and someone who's done it would know.