Depends what you want to do. I'm about to have my garden done so hopefully the following will give you an idea. Our patch is about 70ft square (although its not on a hill).
Clearance of all plants, grass and filling in pond = �2k
40 sq metres of decking = �4k
Fencing (about 110 ft) = �3k
Roll-a-turf = �1.5k
Plants extra
As with most things, the cost is not really in the design or the materials but the labour so your definition of simple may actually be quite labour intensive. It seems most things are measured on how many days they take to complete.
I think the thing to do is to get a couple of people round to give you a quote. Yellow pages came up with lots of landscapers.
What I found however was that once a quote had been given for the clearance, we decided we were going to go ahead with it without getting absolute quotes on the rest of it which has ended up being a bit more expensive that first thought. If I was doing it again, I would get an exact quote for everything - particuarly with the decking as there are lots of variables which effect the overall cost (lighting, height etc) - rather than making rough guesses. To a certain degree our hands were tied because we knew we wanted to get the garden done and within reason, cost came second, but its definately more expensive than we thought it would be (originally thought �9k - �10k inc plants - its looking more like �12k - �13k)
The other thing I found is that our landscaper is a landscaper, not a garden designer - so whilst he has come up with the decking scheme, if you want anything more fancy (built in water features etc) then you may want to appoint a designer first. But again, its probably worhwhile getting a landscaper over first to give their opinion as you may want something within their remit.
Good luck.