Mac Pro - adding third monitor

Hi all,

I currently have these two monitors running from my Mac Pro (early 2009 model), both from the standard outputs including the mini display port:

Acer P223W (1680 x 1050, VGA input)

AOC 2250W (1920 x 1080, D-SUB and VGA inputs)

I have recently inherited this monitor:

SONY SDM-E96D (1280 x 1024, DVI-D and HD15 inputs)

What's the cheapest (and simplest) way to get a three monitor setup?

I'm using it for music production with Logic X, no gaming etc. Third monitor won't be used that much, so don't want to spend more than necessary.

Thanks in advance, any recommendations much appreciated.

Jamie