Prices for Fiestas of that sort of age are from as low as €4,500 up to around the €6,000 mark. As ever it's all down to condition and history after that. 

 

You won't save much by swapping though. A pre-2008 Micra is going to have the same motor tax as your Fiesta and will be about the same on fuel. A 2008 model will be on the CO2 based motor tax system but its 139g/km rating means you'll pay €280 a year to tax it, which is an annual saving of just €70. 

 

To be honest, unless your Fiesta is s total wreck, you'd probably be better off keeping it but treating it to a good long visit to the garage - get every small job it needs done, new tyres etc and a full valet. You'd be surprised how much better, and indeed how much more economical, your car can be when it gets a full 'Spa Treatment' and the cost is almost always less than what you'll spend on trading up.