"We tried to get him last year but they wanted £3million and that wasn't right at that time," Johnson told the Bristol Evening Post.
"But a year down the line sometimes they get more expensive and sometimes they get cheaper.
"He is still going to be (a signing that would need) a transfer fee because he is on loan but we have got the option and we will see how he goes until the end of the season.
"We have already agreed a fee if both parties want to do it."