Asus Xonar DX 7.1 is on sale at Newegg for $75+ $4 shipping - $10MIR = $69.
- PCIe x1 interface? Yes.
- at least 5.1 channels? 7.1
- 24-bit audio output? Yes
- at least 96KHz sample rate? 192KHz
- high SNR (at least 108dB)? claims 116dB
Response: No, I don't have first hand experience with this card, but I do with another Asus (external) sound product, Xonar U7, that's ok. This was the only card that I found meets both your noise level and price requirements. If you relax the noise level a bit, you get much cheaper stuff, for example:
Creative Sound Blaster Audigy FX 5.1 PCIe Sound Card with 600 ohm Headphone Amp
- PCIe x1 interface? Yes.
- at least 5.1 channels? 5.1
- 24-bit audio output? Yes
- at least 96KHz sample rate? 192KHz
- high SNR (at least 108dB)? claims 106dB
- $32.99 + free shipping
In case you wonder, the "600 ohm Headphone Amp" means it has a separate, relatively powerful headphone amplifier onboard, which can drive "difficult" (that is high-impedance) headphone like Sennheiser HD600 (which cost about 10x what this card does, so it's a bit questionable that you'd want this cheap card for those). This feature doesn't affect/matter for the 5.1 speakers output.
And since you're probably going to ask... no I don't first-hand experience with this card either. The last Creative (proper) card I had was an Audigy 2 (PCI); quality was as advertised. I also have an E-MU 1212M, which is their professional brand basically; outside your price range by a factor of at least 2 and not 5.1 etc.