Nathaniel,how good are your websites compared to these £1200 to 1500 ones?
I'd quite happily charge you £1200 - £1500 too, if it makes you feel better?!
Simple answer is that you get what you pay for - I'd expect the more expensive sites to have a LOT more pages than most of the sites I build,.. and since Google loves content, more pages is often a very effective way to get a better ranking,.....
Apart from that, I'd hope that my sites are pretty much the same standard,.. at least in all the important areas.
There's a few examples on my portfolio page here:
http://www.avondhu-internet.com/portfolio.html