As far as price goes...

I would say around $300 - $400 is a reasonable price to pay for either the RG550 or RG570 with a hard case.

I bought a used RG570 ('96) without a case from the local music store here for about 270 Euro.

I also bought a used RG550 ('98 ) with a case from ebay for $275 + $100 shipping to the Netherlands which equals out to 300 Euro.

I have seen them go as high as $500 for a used RG550, but there always be another one for a lot cheaper than that.