Yet people were not interested in buying an expensive cartridge with no or few advantages when the CPC version could be bought for cheaper and played on the Plus.
UK prices: in 1990 when the Plus/GX4000 was released, cartridge games were sold at £39£29.9995, while disc versions were normally £15£14.99 and tape version was £11£9.99. Blank discs were £3£2.99 50 each.(Source Amstrad Action October 1990) French prices: 250Fr for in 1990 when the Plus/GX4000 was released, cartridgegames were sold for 250Fr , 150Fr for disc and 100Fr for tape.Blank discs were 20Fr each. (Fr = French Francs, pre-Euro currency in France).
Most games with CPC and PLUS versions are "Almost Direct Disk to Cartridge Port", a "Direct Disk to Cartridge Port" with small modifications. The use of the extra Plus features varied a lot. Some didn't use much, others used more.