His company also represents Nikki Walker, who has completed a move to Neath-Swansea Ospreys, and he said in today's Herald: "If they don't get the players' values right, others will go. We've got another nine out of contract at the moment and their first instinct is to stay in Scotland.
"We've also got players, including Scott Lawson and Rory Lamont at Glasgow, who are not yet out of contract, but are keen to negotiate longer-term deals. They want to stay but they also want to maximise their earning potential in what is a relatively short career."
Of negotiations over MacLeod, Longstaff said: "The SRU got serious by the end but, if their first offer had been anywhere near their last one, Scott would still be in Scotland.
"When I went back to him with their original offer he said 'Go out and see what you can find me'. As soon as we put his name on the market the response was remarkable. It is a shame he is leaving, because anyone will tell you that he is Borders through and through."
Longstaff continued: "I think I have shown that I understand how important it is for players to remain here for their career development.
"I want Scotland to do well and it's in the national interest for them to be treated properly but we won't be in business if we don't get our players the right deals, and they are becoming increasingly aware of their market value."
Longstaff has been as excited as any Scot by the turnaround in the fortunes of the national side this season which Frank Hadden, the head coach, has attributed largely to a renewed emphasis on the professional teams.
Longstaff remains optimistic that the changes in the SRU hierarchy will make a significant difference, but said he was concerned about the message the governing body was sending out to the Glasgow Warriors and Border Reivers squads.
Since the turn of the year, there have been a succession of big name re-signings at Edinburgh Gunners, Hadden's former side, whereas little has been done to strengthen either the Warriors or the Reivers and there have been several high-profile departures.
"Frank has done a fantastic job in turning things round with the national team and the whole country is grateful to him," Longstaff said. "There were some eyebrows raised when he picked so many of his former Edinburgh players in the squad at the start of the season, but he clearly got it right.
"The SRU now has to make sure it demonstrates that it is not neglecting the other teams, though."