I think in general terms, the ransom note was an attempt to give credence to the idea that whoever harmed JonBenet was from 'the outside'. Some other 'entity' that came and did all the horrible stuff.
I believe the ransom note was wrote by Patsy. The reason I believe this is that most handwriting experts could not rule her out based on many handwriting samples. Some explicitly said her handwriting was that of the ransom note author.
I think specific details are given in the ransom note which are meant to be 'secret' details only a few people would know. John Ramsey was chosen as the protagonist of the note -- in that the note was apparently wrote by someone who had a grudge against him and the country he came from.
The mention of the $118,000 paycheck was clearly an attempt to make whoever/whomever wrote the note appear as someone who knew private details about John Ramsey's life. Further, the specific number of $118,000 as opposed to say $1million is pretty much designed to give the impression the person was 'annoyed' with John Ramsey and jealous perhaps -- they knew of his $118,000 bonus.
The fact that the ransom note author specifically mentioned that bonus must reveal something of their inner machinations -- if Patsy wrote the note as assumed, clearly the large sum on John's bonus was a source of happiness/pride/ in a way, so much so that she felt it would be wise to include it in the ransom note to imply 'others' were jealous of it.
Basically, the ransom note tried to establish a motive for JonBenet's death in my opinion because all that existed in the house was a dead, sexually molested body.
But yet the FIRST thing JR said when he laid JB's body on the floor was "this is an inside job". I have always felt the Rs conspired to pin this on housekeeper LHP. She had a key, was familiar with the family, was financially stressed and her lower socioeconomic status and lack of education made her a perfect 'patsy" (NO pun intended).