A visit to a second hand carpet warehouse in Croydon earlier this year allowed me to purchase a large number of carpet tiles removed from an office at about $1.50 a tile. This is good value and the fact that they are tiles will allow for replacements should any be damaged during construction or should one of my operators spill his beer, sorry, coffee. It is also a lot cheaper that buying new carpet.
The carpet warehouse I visited had a number of different patterns available in varying condition from almost new to down right disgusting. Fortunately my purchase coincided with the steam clearning of the carpet in our home so I was able to get all tiles cleaned as well for free.
So it's just a matter now of sorting the good from the not so good and laying the best tiles in the visible areas and the not so good ones under the layout.