How much a plumber charges is determined by the scope of the repair or installation. Before you hire a contractor, get a quote for the job, including the cost of parts and any additional charges (is it a one-off price or does the plumber charge on the hour?).
It’s also a good idea to contact other professional plumbers and get multiple quotes. This will help you negotiate a better price. But do bear in mind that experienced and more skilled plumbers most likely will charge higher than someone who isn’t at their level. Like the old saying goes – quality comes at a price.
Most plumbers will be able to give you an estimate for the most common repairs and installations like faucet repair and boiler installation after going through several questions over the phone (for example, is it a mixer tap? What boiler brand do you want?).
Another factor to bear in mind is the location. If you live in London, for example, the prices will be generally higher than somewhere up north.
So to conclude; get a full comprehensive quote, compare quotes from at least 3 plumbers, and also look at the plumbers skills & experience. This can be determined by how long they’ve been a plumber, online reputation, company registration date and so on.