Check out ebay for some Amigas.
A500 usually sell for around £20
A600 about £30
A1200 about £50-£70
A2000 about £35-£40
A3000 upwards of £100 i think
A4000s can cost upwards of £100 depending on hardware
Not sure if you know about the new power pc Amigas that run at 800mhz? http://en.wikipedia.org/wiki/Amigaone