What Tony said....Gulfstreams are top of the line aircraft and often an older Gulfstream is a better airplane than a newer something else...
That said...Operating cost is usually more important than acquisition cost. Especially since there are so many incentives to buy new aircraft. Consider warranty, training costs, sales support, etc.
The biggest problem with new aircraft is they are difficult to get. Back orders, delays. Often people order new Gulfstreams just to flip them on delivery for a nice profit.
A late model second hand aircraft may be the best option.