For a decent mk1, you'll be looking at £2k for a well looked after one - a Mk2 about double that.

But of course it depends what engine you go for - 1.0litres are cheaper, 1.4s are more expensive.

Piece of advice - watch out for 'S' trim Arosas being sold as 'Sports'. A true sport will have a centre exit exhaust. :)
 
Does she want a MK1 or MK2?

Mk2s are often 1.0 engines, seems the most popular choice, and they are great little engines.

Mk2s were only available in the 1.4S manual for a short time i think, they arent as popular as the 1.0 as there is only a 10BHP difference. Most 1.4S models are automatics.

Then there is the 1.4 Sport, which needs to be worked well into the rev range to get the best from the engine.
 
I've had my 1.0 Arosa for nearly 3 years and have had absolutely no problems with it *touch wood*. It's very reliable and a very cute little car!