I'd say £4-£8 for a weekend bimble destination: £100 bacon sarnie scenario.
£10-£15 for a jolly to a particular destination (ie, stopping overnight, going to the beach, lunch at a good pub etc).
£10-£20 for a business trip to an airfield with good facilities and close to my destination. I'd expect 4-6 hrs parking (on grass) for free, and I'd be happy with paying £10 overnight.
If I was heading for a major airport in "any" weather and wanted to know that I'd be able to get in (and the company is footing the bill), then I'd be happy to book in and pay up to £30 (no nasty extras) for the privelege of a guarenteed safe approach.
An NDB for that money is taking the piss though. So would a simple ATIS/App/Twr service - I'd expect radar vectors/monitoring as well!