"I have some money available, but can't have some players on low salaries and others on high salaries."
Boyd is currently on loan at Nottingham Forest and has one year remaining on his contract at London Road.
Posh are set to cash in on the 24-year-old and will expect to receive a seven-figure sum.
However, with crowds at London Road rarely rising above 9000, Johnson has said he cannot afford to unbalance the squad by paying high wages to a star player.
Instead, he will be aiming to bring players in from more affordable sources.
He said: "It's not as easy as someone turning up on your doorstep. You have to know who's about and know the young lads who are coming through.
"You've got to be out there looking. You've got to be going to games and you need friends to recommend players."
Meanwhile, youngster Carl Piergianni has signed a one-year development contract, but Charlie Dove, Alex Austin, Edward Adjei and Sam Cole have all left the club.
Source: BBC Sport
Source: BBC Sport