DVI output to VGA input is pretty cheap.
VGA output to DVI input (or HDMI input) is a bit more expensive.
I got one for around £50 - it also takes composite video and converts it to DVI / HDMI.
The quality is not good. Usable for movies, but pretty bad for computer desktops / consoles.
IMHO getting a monitor that has both DVI and VGA input is a better (and maybe cheaper) approach.