I guess that depends on where you are and what you are paying with. In the USA a smart shopper can get an i7 870 8GB RAM HP desktop, 1 - 1.5 TB HD, win7-64 and a crappy graphics card for about $600 as a refurbished unit. You need to add a couple hundred dollars for a decent graphics card and almost that much more for a nice widescreen monitor. Maybe $30 or less for the Logitech joystick with throttle and a dozen or so button and switches. Later on a $20 headset for Christmas. If you spec something out make sure you get a good wheel mouse - L/R button w/Center roll wheel. Doesn't cost much extra but it is well worth it. A downside of these HPs and Dells is that you can not overclock them so you would want to have something around 3.0 GHZ or so as a minimum. Dell.com has stuff on sale all the time. They also have an outlet that you have to check almost daily. You have to pay tax and shipping here unless you have some special discount code (which comes and goes) Ray

What would an average desktop computer cost? One that runs FSX fairly decent.

Heavily depends on what you're looking for and where you live. In the Netherlands, you can get yourself a capable system excluding a monitor for around 700 ~ 750 euros.