Jackson has proved his credentials in the lower leagues over the past two seasons and recently expressed his desire to be given a chance at a higher level.
Premier League side Wolves have been linked with a January bid for the 22-year-old, but Stimson insists the Canada international will not be leaving Priestfield for the sums being quoted.
"(£1.5m) would not be enough money," said Stimson. "If someone got him for that it would be a snip.
"Whether the chairman's [Paul Scally] view is different, I don't know. It might be less, it might be more.
"I'm told we don't have to sell him. Every player has got his price and all clubs need financial gains.
"I'm desperate not to lose him. If we do decide to sell him, that's a conversation I have to have with the chairman.
"There's going to be a lot of speculation about Jackson between now and January. He's just got to focus on his football and whatever happens will happen.
"All I'll say is enjoy him because we don't know how long he is going to be here for."