Going back to the OP’s question – depending on toilet it should just be an hour’s labour plus parts, so about £50-70 plus £20 for new syphon.
Problem with forums is that everyone’s an expert and will say you can do it yourself following youtube and if you don’t you’re not a real man.
…and you can..as long as you have the tools to cut off the seized wing nuts, a replacement doughnut washer that fits, tape for sealing joints, silicon etc…oh and helps that the plumbing supply place is open on Sat pm and that the previous owner hasn’t boxed in (and tiled over) the isolation valve to the toilet…so you have to turn the water off to the whole house for two days while you get stuck into the above..
Other than that, it was easy 🙂
And who the hell invented the syphon where you have to take the whole cistern off the wall just to change a rubber diaphragm 👿