Niall,
I gather both would depend on how its done. I agree that adding a back tee, and making them play over unirrigatied natives really means a few small sprinkler heads. Not even much turf around the tee is necessary. And I doubt those back tees need to be any bigger than 20 x 20 feet, maybe even 15 x 15 feet. That is what I did at Firekeeper and I don't think the 7400 yard tees added much cost at all. I didn't even take the cart paths back there, since I figured about a dozen rounds a year would be played there.
Width is governed by sprinklers, so if you can upsize existing nozzles and avoid adding another row of sprinklers that might not cost much either. If you are doing a new irrigation system at the same time, you would more likely consider going from 2 to 3 or 4 rows, maybe 5 or 6 in the desert. If you widen the fw, it might cost some money, depending on re-grass, add sand cap, etc. Rough is pretty cheap, usually.
But, overall, if you add length, you make it harder for 0.1% of golfers, if you make it wider, you make it easier/more fun/strategic for 99.9% of players. Even if its slightly more cost, it looks like pretty good math to me!