Agree with JonFairhurst... I found out that M-Audio has a sound card with no bells or whistles, though it's GSIF compatible and has S/PDIF and analog inputs and outputs too.... 99 bucks. It's called the 2496 Audiophile.
The main advantage of the MIA-MIDI over the Audiophile 2496 is the the MIA has balanced I/O for lower noise. It also supports four virtual outputs. This means that Giga can render four stereo outputs at a time with the MIA-MIDI. Giga can render two stereo outputs at a time with the AP 2496 (one to the analog output, the other to the digital S/PDIF output).
Other than the balanced I/O and virtual outputs, the cards are fairly similar functionally, and both sound good. Better monitors would have a much bigger effect on sound quality than higher-end soundcards than these.
For more I/O and different formats, or for better A/Ds for pro-recording, you would want to move upscale though.
If you really want to go cheap then use a Soundblaster Live card. I picked up one at a computer show for $4. Then go on the net and pick up the free Project KX drivers (do not use Creative's Soundblaster drivers). the KX drivers are great, they are low latency GSIF and ASIO. One thing is that you will use the Rear set of speakers for the output jack on the card (not the front set (minijack next to the D15 conector)). By the KX drivers using the rear set they use the better of the two converter chips on the Soundblaster. I have an extra box running like this with the output of the soundblaster going straight into my analog mixer so I can tweak the sound easily before bringing it into the main box. This is a very cheap way to build banks of Giga boxes. An example of a cheap box would be cheap standard ATX case with power supply $30, AMD 1.8ghz Duron $40, PCchips Nforce 2 chipset motherboard $10, 1 gig Corsair PC3200 value ram $80, heatsink with fan $15, used Soundblaster Live card under $10, Get a used Matrox G450 or G4+ video card (they are $15 to $25 on the net or $10 at a computer show and they are dual head - very low latency (won't hog your PCI buss) - they have solid drivers and are one of the best video cards for music apps (they run all but the latest DirectX 9 games great also (can only do up to DirectX 8), add a couple of IDE hard drives . . . you get the picture. Check Newegg - Monarch - ZipZoomFly - Chiefvalue for cheap prices from reliable online vendors. You can have a Giga box for around $300, add a KVM switch so you can use just one set of Keyboard - Video Moniters - Mouse so you do not need to re-buy those items, and you will need to buy the gameport joystick/midi interface cable $20 to get midi to the box. If you are building a bank of cheapo's then pick up a used MOTU Midi Time Piece AV (parallel interface) don't know what their current used market price is but it can't be much, with it you will have an 8 x 8 midi interface for your multiple Giga boxes. Bring all the outputs into a mixer or submixer back to your recording box and you now have the ability to do a whole orchestra at once on the cheap. Good luck with the new box