Depends on how "into it" you are.
If I was really pursuing music on the mandolin (in a band, playing for some years, etc.), I'd probably be looking to buy a "very good" instrument in the $1200-$1500 range (used). That's just what a good instrument goes for used. My banjo was $1750 used. New, it would have been $2500.
It's like buying a good Martin guitar.
That being said, you could probably get away with a used decent/good instrument $750-$1000 (decent tone, good playability).