I had a 'custom' exhaust system that was for a Mk2 Jetta GTi 8V. It was a second hand *******ised aftermarket Corrado system which was linked to fit, and it really didn't work well. This was pre-catalyst days so had none of the cat issues to content with. The car lacked torque as there was insufficient back pressure in the system, it sounded dreadful and to be honest was just a mess.
Although its easy to think that an exhaust is nothing more than a pipe to expel engine gases, and the bigger it is, the more performance you'll see top end, from my experiences it seems that there needs to be far more scientific approach to maximising flow rates within the system, controlling the back pressure levels retrospectively to the engine application, and then a really good company will tune the exhaust to give a decent soundtrack in the appropriate rev ranges.
If you can't afford to buy something aftermarket, then don't buy at all (or buy second hand), as you may end up wasting what budget you've got on something thats not fit for purpose. There are plenty of people on here who may agree with this, based on claims that were made relating to a 'custom' system for the LCR which turned out not to be all that was hoped for. If it was easy, eveyone would be doing it.