The size of the job can vary.
For the boiler change you need to engage a heating engineer to quote you.
Bear in mind the price will depend on the boiler that you choose, which depends on what heat output your house/flat needs and the quality (or lack of it) that you want.
For the rest of the system, it depends on the age and condition of it. The increased pressure is capable of exposing many latent faults and weaknesses, which may entail extra work. As a bare minimum, a competent and thorough engineer will flush the system to prevent existing sludge and debris from shortening the life of the new boiler.
Whether you then pay for the removal of the redundant parts of the old system is up to you - you could ask your installer to make sure that they're safely isolated, then remove them yourself when you feel like it.
In short, the forum isn't really aimed at answering your question, but you may find some folk will give you a guide price. Far better that you engage a local heating engineer (NOT plumber) that you trust, preferably found by word or mouth recommendation, and by all means come back here to confirm that acceptbility of his proposed of boiler.
Also, bear in mind that the installation work is notifiable under the Building Regulations, and that whichever business you engage to do the work is required to be CORGI registered.