Well, sticking with what you have is probably best, because of all the units I know of, they all cost $200 plus. These are things like the Mackie Big Knob or the PreSonus Central Station. Both of those actually do more that what you want which is why they cost more.

If you're handy, I'm sure there has to be a DIY solution out there somewhere.

did someone say DIY ?
8)

as has been said, they can do more than just the volume
Dangerous ST Mon at the most expensive and very good
to
DIY for the simple and not so simple
Texas Volume chip PGA2320
... see my site and do the google thing
lots of options

I though you might chime in on this one...however I don't really follow that Haiku-ish response.
:)

You could probably get away with using a simple passive attenuator. Are your sound card outputs and the monitor inputs balanced or unbalanced?

Some possibilities to investigate:
http://www.shallco.com/frameset.html
http://www.siteswithstyle.com/VoltSecond/12_posistion_shunt/12_Position_Pure_Shunt.html
http://www.suertenich.com/html/audio/stepped_attenuator.html
http://www.diycable.com/main/product_info.php?cPath=31_71_77&products_id=274

theyre balanced, DIY is a possibility, but i thought that they'd make something cheap that does it! is there some sort of step by step guide on how to make something because im not too great at any of that stuff, and i want something reasonable.

You could try these kits:
http://electronics.dantimax.dk/Kits/Selectors_-_attenuators/index.html
The RelVol3 is the balanced version.

yep that one is from Mikkel Simonsen and I currently use it in a Headphone Amp unit
obviously with the remote control ... see the dawmonswitch_3.htm page

Mikkel is very helpfull

Try an SM Pro Audio M-Patch 2 for about $150(US). Soundcard speaker attenuator, "patch bay" for other inputs/outputs, and headphone amp. Works great and, you don't have to build it!

TG