Most servers have a couple of unique scripts that actually require time to convert, but most of them are rope, potions etc that you can just copy paste from the repo.
It's imposible to say an exact price on these things, but I have charged from 50eur (a really simple server, with very few custom scripts) to 800eur~ for a server that had a crap load of custom things.
Also depends if the person wanna keep the old monster pack etc, there are alot of things that you have to calculate with porting a server to another version, you can't just upload a datapack and ask for a price, you have to actually say the things that are custom (can we ex copy paste the monster pack (talking about some rl map server) or do you have 100+ custom monsters?)
You can also in alot of cases just run the server via the compat.lua file, but it's not the best thing to do tbh, it's like you wanna say one thing but you have to say another thing before you can actually say the thing you wanna say = extra crap you don't even need.
And since the server is created from 0.3.x you have to add alot of functions to the compat file, but you still have to rewrite certain functions, ex. the function onStatsChange got split into 2 function, onHealthChange and onManaChange etc
So my tip, send a pm to some ppl with what you actually want done, ask for an aprox. price and see who you wanna go with.
Remember that the cheapest guy rarely has any experience, while the guy who chargest the most might know what he's doing, but might write the exact same code as the guy charging 20% less
