I've listened to a lot of brands of FM-Modulators, and they all pretty much sound satisfactory. The only thing I would suggest is get one that hooks into the antenna (as mentioned above) and not a wireless one. I have yet to hear a wireless one that is close to satisfactory. I would expect to pay around $40. You could find one cheaper, but you get what you pay for.
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Mark IIa -- #30103119 -- 70 gig w/ ext3 fs-- Eutronix Light Kit -- Greenlight buttons