Headlamp cover replacement?

If you mean the clear portion of the lamp, that isn't a replaceable part. You would need to replace the complete lamp assembly. They are bonded together with the reflector assembly as a sealed unit. Pricing wise they can be very expensive depending on the actual lighting option you have and which lamp assembly you need. The Fiesta didn't become available in that version until 2011 and there was a facelift done in 2010 so the part and price I have wouldn't match your car but for the light on the 2011 base model (which is still similar in size and shape) is $340.00 so I would expect that would give you an idea of the price at least. Being a 08 you will likely need to source one used which should lower the price.