Actually from my experience its normally just the firm rather than a middle man.
Its often caused by the government having a very restricted list of approved suppliers and not being able or willing to go outside of that list., The Suppliers know they only have limited competition and so can in inflate their prices and the fact that the person making the decision often has no or very little knowledge of what the cost should be
The mindset (from the top) in most Governments is
X – Approved supplier
Y is a new supplier not “approved”
Civil servant uses X company (and that costs £1M) and project is a success – That’s you job
Civil servant uses X company (and that costs £1M) and project is a failure – not my problem I used the required supplier
Civil servant uses Y company (and that costs £750K) and project is a success – Well done BUT no better well done than using X* and in most cases not recognised as saving £250K
Civil servant uses Y company (and that costs £750K) and project is a failure – its all his fault
The reason that saving £250K is not rewarded is that a business case would have been done and the approval to spend the £1M or £750K is irrelevant as far as the project is concerned. Now there will be projects that fail the Bus case at £1M and would have passed at £750K but the Civil Servant running the project is not affected by that decision