A d-hoc jet charter is the best option for lighter aircraft users, particularly those who fly less than 10 hours annually in a private jet. A "Block Hours" program may be considered if you need to fly 25 to 50 hours per year - and you won't have any responsibilities of ownership...
This "jet card" type membership is available from a number of aircraft operators in the US and Europe. Block hours means literally that - you purchase a block of flying time (normally 25 or 50 hours) on a specific aircraft for a fixed upfront fee. On top of that you may also have to pay airport fees/taxes depending on where you travel and possibly additional fuel surcharges and insurance fees.
One of the plus points of block hours programs with major operators is that you can often upgrade - at additional charge - onto larger aircraft types if your needs change. Obviously the downside of block hours programs compared to jet charter is the need to pay the large fee upfront. Also be aware of any cancellation fees if you need to cancel the flight at short notice, say less than 24 hours.
Netjets (better known for its fractional ownership offers) has a well known block hours program known as the Marquis Card in the US. This costs from $119,900 (prepaid) for their smallest jet aircraft, the 7 passenger Hawker 400XP. Aircraft are available world-wide from a minimum of 10 hours notice and you don't pay any positioning/empty leg/airport fees.
This is effectively a base rate of $4,800 per hour - to this you must add a fuel variable fee of around $865 per hour (which changes monthly) plus other taxes and charges.
If you needed a larger aircraft (10 types available) such as the Citation X, the total hourly rate including fees would be around $12,200 per hour. Netjets has a presence in Europe and clients can use aircraft on both sides of the Atlantic if required.
Other major jet card (25 hours) programs available in the US: FlexJet by Bombardier; Citation Jet Cards (peak/non-peak options); Flight Options Jet Pass; Sentient Jet Card.
Jet Card Programs in Europe/UK
Air Partner has a 25 hour "JetCard" program - annual charges range from a very light jet (4 passengers) at €108,000, a midsize jet (7 passengers) at €186,000, right up to a large global jet (14 passengers) for €385,000. No additional surcharges are charged, flight time is wheels-up to wheels-down plus 6 minutes taxi time at each end and a 15 % discount is applied on day returns.
BookaJet's Jet Black Card (25 hours) costs €125,000 for a light jet (6 passengers), €178,000 for a midsize jet (8 passengers) and €277,000 for a heavy jet (14 passengers).
Other jet card programs in Europe: Netjets Europe Private
Jet Card; VistaJet, which offers block hours from 50 hours; European
Skytime "Flight Commitment" program; Blink "Foresight"
50 hour card (very light jet).
Many of the companies offer carbon offsets as part of their program - this will add around 2% to the base fee.