I purchased Dragon Slayer brand new in 97and it did not come in a cardboard box. It was the first game i ordered from Radio Shack. It doesn't seem to be one of the really expensive SCD games that were released in the US, but it does seem to be fairly popular. I've never made a mental note of the ebay price these days because i own it, but i would guess between $30-70 dollars for a complete copy.
One thing i would like to bring up is that i've seen some later period turbochips like Nuetopia 2, Air Zonk and Dead Moon being sold with jewel cases. Does anyone know if there were some copies released this way, instead of just the cardboard box and plastic tray? I saw one copy of Neutopia 2 awhile back on ebay with a jewel case that included a back panel with text, screenshots and everything. Does anyone know more about this?