Unfortunately, I haven't had a custom exhaust made for my car, but I have however done a lot of research into them and after ringing a few companies I realised it wasn't as expensive as you'd first imagine!
What I found out is as follows:
- You can have one made for between £300 - £600 depending on what kind of performance/sound and characteristics you're looking for - (Of course you could spend more for Titanium exhausts, Cat-back systems etc, but this is the general pricing).
- This includes fitting the exhaust too, in most cases.
- The majority of companies will have your car in to measure it up to ensure they're going to build something that will definitely fit your car instead of just taking your model reference down which was a reassuring aspect to me, maybe hassle for others.

- Most companies often offer between 2 year and 5 year warranties.
Exhausts are again, another upgrade where you get exactly what you pay for, and if you're looking for one to last you at least as long as you're going to keep the car, the investment is by far worth the extra £50, £100 or £200 depending on your budget.
Note: This information is based on my own research and could differ greatly depending on the companies you contact, the distance you are from these companies and the reputation which they have generated for themselves. This is where I saw most difference in price as the more prestigious companies would often charge more for a similar if not identical system as other companies would provide. Designer exhausts comes to mind...
Hope this has helped, Olly.