Remember that the later model Astras have a MP3 compatible headunit.

A vastly cheaper option would be to make up compilation MP3 discs and use them in the headunit. 3 or 4 CDs kept in your car would give you alot of hours of music. Seems a bit silly to spend $100s just to connect a cheap MP3 player to your cars head unit.

The best option would be a USB connector. Load music onto a cheap USB key and have the head unit read from that. You don't have to lug an expensive ipod etc around.