I was given one by my friend. Long story why but I haven't built it yet. I did help my neighbour put one in, he didn't know what to do and phoned a few companies and they were charging a fortune so he asked me. I am a builder by trade, have done all sorts of work in the construction industry though so although I'd never done one before it was pretty straight forward.
What might make your costs high is tools. You can hire them from hire shops but they charge a fortune too, but again, I had most of what was needed. I would really watch who you get to do the job if you choose someone as there really is far too many people out there trying to make a quick few pound and then when the job goes wrong they want extra money to repair it.
As with fish, do you research first, work out exactly what it would cost for you to do it including tools, building materials and the time and effort you put into it. Then see if its better for someone with experience to do it.
