This is a common question and the common answer is as much as you want. There are lots of considerations when purchasing imagery but in general satellite images are cheaper than aircraft, low resoltuion images are cheaper than high and old images are cheaper then new. To get some idea , you can look at the cost per sqkm of newly aquired imagery to get an idea of comparision.
Worldview 2, 50cm pan is about €30 / sqkm
IKonos pan, 0.8-3m resolution is about €25 /sqkm
Deimos -1, 22m res is 15c/sqkm
Landsat of course is free as are the MODIS and MERIS sensors
A high qulity ariborne lidar survey would be in the order of €450/sqkm.
There are a lot of ways to analysse cost (e.g. per pixel worldview is much the cheapest of the three listed above). Also note as price per km may be quoted but you will ofetn be obliged to have minimum order of a few hundered sqkm – which may squew PROJECT costs back toward airborne if you are only interested in a small area.