Do you think you would likely get a higher offer?
I'm not familiar with him (not a Padres fan) so I don't know how good/bad he is. With the Padres 11 games back of 1st in their division and 50 games to go are they likely to get any better? (Making the demand a bit higher for it at the end of the season?) On the other hand if they begin to tank later offers might be lower.
I probably would take the offer. (I wouldn't buy a card for 100 bucks myself but selling it for that much is a different story)