Have you an engine in mind?
Any respectable engine dealer will supply and fit the outboard for you and you should, in my experience be getting some change from the RRP of the motor, should you strike the right deal. If they are supply only, then you should be looking at a heavily discounted price for an outboard 'in the crate'. Take care with warranty issues on a new engine not fitted by an authorised dealer - some have specific set-up procedures which an authorised dealer must go through (or so I'm told).
On top of the basic outboard supplied by the manufacturers, you will also require engine oil (2t or 4t), a battery and box, battery leads, fuel tank & line (on larger motors), control cables and perhaps a different control box and/or propellor. These bits are reflected in the rigging cost by the supplier, which will also include cutting out the console and fitting any instruments, etc. included with the motor. On more complex modern motors, engine fitting and set-up will be far more technical and time consuming than in the good old days.