I bought my walker 2nd hand for £250 but with ground works for the base, fencing materials, electrics etc and a wood chip surface I think it cost me about £3.5k. We did everything except the base ourselves and it is only around 9m in diameter so I think this is about the absolute cheapest you could do it for unless you had an existing hard base.
We have a Monarch with auto reverse, electric pushers rubber pavers and mesh fencing - it was well over £10k excluding the base. Monarch were very professional to deal with.