It really depends on what you want to end up with. It's quite possible to add the controls for <$1000 on the low end and have a reasonably serviceable CNC machine. Or on the high end, the sky's the limit. I have done 2 mills for about $7500 each. And currently have a lathe in process that will be around $10,000 when completed. The two mills were already CNC machines as is the lathe. The lathe price includes all new servo drives and motors, the mills used the original drives and motors.
Your BP would probably be the easiest to convert because there are bolt in ball screw kits available, not sure if the same is true for the Excello.