Akrapovic is crazy expensive, which comes with the territory when you specialize in porsche and Ferrari parts. Raw materials for a titanium exhaust should run $6-700, and forming a custom exhaust system isn't that difficult for a decent shop. Just make sure you find somebody comfortable welding titanium. $2-4k for an exhaust is ridiculous.
A titanium exhaust would weigh a bit over half of stainless. I can't find the link, but I think I read that the OEM catback is 33lbs, and whichever aftermarket system being compared was 27lbs (if anybody remembers which thread, a link would be great). A stainless setup will run ~$6-800. I would throw an extra $200 at an exhaust if it would save me an extra 15lbs.
As light as the Fiesta is, any additional weight savings will be valuable for trimming those tenths at an autocross.