Dear Mr Chime.

This is the cheapest way you will get.
First off find a PC running XP that has a parallel port, install Mach 3.

Then buy a breakout board like the one you linked to but this one is far cheaper.

http://www.ebay.co.uk/itm/401153797580

Then buy 4 of these/

http://www.ebay.co.uk/itm/371436535450

You only need 3 but buying a 4th means you never have to wait if it breaks down.

So for a total of a SH computer, and £21.14 you will have everything to convert your machine if you use the existing steppers and power supply out of the Roland.

Both these Ebay links are from the same seller BTW