I don't think £7k is excessive, and here's why: I asked the bloke at that stand whether that included labour, materials, plants (raised from seed or bought mature?), trees, and all that, and he said it did actually include everything other than the chiminea, table and chairs, and the sculpture. Clearly there would be a difference in the labour costs to do that garden in (e.g.) Notting Hill vs Doncaster, so there is a bit of a fudge there. But it did include about a week's labour, skip hire for excavated earth, some reasonably mature plants (e.g. the four trees were £700 total), the reclaimed (but not free) materials like the scaffold boards, etc etc. Also, the garden is shown at approx two years after doing the work - so plants are put in more mature and abundant than the initial planting would be for the money - another artifice to get below the £7k measure! He also didn't mention the cost of the designer - think of the time taken to design, source materials, project manage and guarantee that garden for the client, I have no idea how many gardens like that the average designer/landscape gardener can push out in a year, but if it's even only £1k a time, you need to be completing one about every ten days (taking off taxes) to just be on the average UK income! I think perhaps the RHS is (intentionally or not) making a subtle point here that garden design is high skill and artistry for actually very little money reaching each worker in the team.
If I was to honestly calculate what I have spent on my garden I think I would need to have a bit of a lie down. But in terms of added value to your home (by which I don't necessarily mean re-sale value, but the emotional value of having lovely, personalised, fragrant and colourful surroundings), £7k for a very designy garden on a £200k house is about comparable with the complete cost of fitting a new kitchen. If you are a family of 3/4 in a cute little terrace house with one reception room at the front, spending £7k to make the garden an alternative sitting area to read or have meals is like creating another room - that's certainly worth it!
I think that the show gardens in all categories are on a different level to normal people's gardens. They're a concentration of elements that in reality you would spread out much wider or take a few from in the same area.
OK, I'm tempted to be very honest now, I'm going to work out approx what my back garden has cost (or should have cost in the case of free labour / free divisons of plants) in two years. Or you could look at it as what it would cost to recreate if it was totally trashed by a tornado or something. It's about the same size as those Low Cost High Impact ones...
Decking £1200 inc skip hire taking away earth and notional cost of my dad's labour
Climbers (8) at about £10 each, £80
Herbs (about 30 plants?) at £2-5 each, £150
Flowering/shrubby perennials (lavenders, dicentra, sarcococca, heucheras, euphorbias, astilbe, astrantia, sisyrhinchium, dogwood, skimmia, hypericum, euonymus, monarda, hellebores, lilies, dianthus, alchemilla, achillea) £300
Roses £25
Dwarf fruit trees £60
Pots of many sizes, wooden and terracotta, 8 large, 20 small/medium, yikes about £350
Edging around the lawn, about two tonnes of large stones, £250?
Rotavator hire and a tonne of topsoil to prepare the lawn area, £150
Pond digging and rockery building, maybe two man-days of unskilled labour, plus a tonne of stones and a breezeblock wall to back it, £300
Acer on the rockery and lots of alpines and bulbs, £100
Two sheds 
plus four man-days digging and concreting the slab bases and building the sheds. £1500
Various durable veggie grow-bags, £50
Veg seeds, onion sets, seed potatoes etc. (and young plants on a whim to fill gaps!), £50 (per year, realistically)
Compost to improve beds and borders (before home composting /leaf mould gets going) £250
Stakes, cane supports, string, £50
Mini grow house £20
Grass seed £25
spring/autumn bulbs £50
Fence around pond inc labour £150
Compost bins and water butts £150
Wheelbarrow, lawnmower, strimmer, hedgecutter £200
Tools £200
Seed/container compost £60 (per year)
Hanging baskets £30
Ericaceous compost £15
Tables and chairs £250
total £5995
I've tried to be very honest there and not underestimate the value of things. I think the total is probably fairly accurate in value (not cost) although the obvious omission is my own labour! I might take a look at my insurance docs tonight, just in case we do get more horrific weather...!!