Smart homes can be affordable if you shop around. If you decide to choose the best smart home system in the UK with all the features you can imagine, it might get costly, but it really depends on what it is you are looking to automate. If you want to just link up your entertainment devices, it can be quite affordable.
You can also now get smart lighting and thermostats relatively cheaply. You should shop around for installation and try to find a home automation company that is offering a good price for installation. It's only when you go for the whole hog that you will find it becomes relatively expensive.
You should sit down and really think about what features in your home you want to have automated. There's is no point paying for devices you are never going to use.