The costs will vary from property to property, but you should be able to get a complimentary consultancy to work out the costs of the system and what RHI will be available to you. In most cases for 3 to 5 bed properties, the benefits can make switching cost neutral and even profitable. However, if your property falls outside of this range is may still be beneficial.
The system itself can cost anywhere between £10k and £15k and most customers set up an easy to manage payment plan, you can also claim up to £13k back from the government as part of their Renewable Heat Incentive which is paid quarterly over a 7 year period.
You can read more about the Renewable Heat Incentive here