Yes I have, for an Alaskan service though, rather than a bandmill.
I charge £300/day for milling. This is an all-in price, which I have worked out around costs.
Green hardwood can reasonably be valued at £20/cu.ft. This is a bit arbitrary as oak is about the only timber commonly available green, but £15/cu.ft is the lower end and bear in mind that £20/cu.ft is effectively a delivered price, so it's about right.
The timber can be presumed to be free as it will typically be single trees which already belong to whoever wants them milled.
It depends a lot on the log and the spec but in a day I will typically produce between 35 and 70 cu.ft. which would have cost £700-£1400 to buy. This represents a very significant saving. I will then typically end up leaving it stickered up properly to air dry, after which the equivalent purchase cost would be significantly greater.
However, what I always discuss with the customer before taking a job is whether they actually have a use for the wood. Some have clear ideas for it, such as building large sheds/summerhouses, or a new floor or dining table. Others intend to sell it - I warn them this is likely to be challenging and require time and effort on their part but whatever they decide they want to do with it, in the end they are the customer so if they want me to mill for them, I am happy to do so.
Those who have a clear use always come out with a significant saving over buying the equivalent timber, even if they had bought treated softwood.
Those who don't, or intend to sell it, may not. The one exception is when I am milling small diameters of short lengths, where the set-up time outweighs the value. This is only really worth doing when the customer has a sentimental reason for doing so.
Alec