At our garden centre we charge between £5 and £20 to refill hanging baskets. Or between £5 - £30 for brand new completely filled baskets. We charge less than this for volume orders to local pubs, resturants, shops, nursing homes, cafes, etc. there is at least 2 other places within 10 miles of us that also offer this service.
Just to give you an idea:
our £20 baskets contain 21 plants that cost between £1 - £1.49 each to buy at our centre. Hanging basket plants cannot be grown from seed as they are not true to type so they have to be grown from cuttings. most of the cuttings have plant breeders rights so loyalities have to be paid to the breeder for every plant sold.
Then our baskets have a liner that costs between £1.50 - £3.
We also use the very best compost £6 for 60ltrs (which makes about 5 large baskets)
we add water retaining crystals £3 for a tub that will do about 5 baskets.
we add slow release fertilisers £4.99 for about 15 baskets.
So really If one of my customers was to exactly replicate one of my baskets it would cost them: £21 for plants (cheapest ones) £1.50 for a liner (cheapest) £1 compost, £1 feed & water retainers = £24.50.
We grow the baskets on from late March for when our customers require them, which in our area is normally mid may to be frost free. so we also have a minimum of eight weeks of watering, liquid feeding and deadheading whilst paying rates on the area these baskets are stored and believe me they take up a lot of space.
Work it out for yourself based on local prices what you could buy the stock for etc. You may find a local nursery will be prepared to do all of the work for you, but then your customers will probably use this nursery too and cut you out.
sorry not trying to beat down your idea but judging on one garden centre (part of the most expensive chains the garden centre group) which is "possibly the largest garden centre in the UK" (their claim) really isn't a good indicator of price they will most definately be one of the most expensive places to buy these sorts of "service" products .
Off topic but I certainly wouldn't want to see their overheads, their wage bill must be astronomical!.
http://en.wikipedia.org/wiki/The_Garden_Centre_Group
and further off topic:
Lloyds bank lost 100million this year selling of the garden centre group at a loss to terrafirma.
http://www.telegraph.co.uk/finance/...-as-Terra-Firma-buys-Garden-Centre-Group.html
but this kind of gives you an idea of why they are so expensive - they have to be.
And I've started .... hosepipe bans in this area really affected basket plant sales this year. most people have opted for pots rather than hanging baskets because being on the ground pots are easier to water than hanging baskets with a watering can. Now people are thinking hosepipe bans are over its far too late for summer baskets, we start on winter baskets in three weeks time