The cheapest external A-D that can feed your s/pdif input are found in hardware boxes like FX units. If the box is 24bit/96Khz capable, then the converter is probably at least as good as the ones in your soundcard. For some peculiar reason, this kind of solution is usually far more cost effective than buying an external ADC unit.
With a digital FX box, the method would be to set up a dry effect mix, although you might want to take advantage of any dynamics processing it has, but remember it's next to impossible to remove all the effects of compression from a recording if you later decide you don't like it's sound in the mix. You get extra bang for your buck if the FX has a better reverb than your DAW, which you can include in your mix from an aux send bus.
A down side to FX units is that they normally, probably always, only run at one sample-rate. The Delta card has to be set to External Clock in its control panel Hardware Settings tab to use the s/pdif input, that it, is has to slave and lock to the external s/pdif inputs rate, so your project must be at the rate determined by the external box.
Turning to units designed mainly as external converters, the cheapest also have output DACs that can run off a cards s/pdif output. That I know of, there is...
ART Dio
Behringer Ultramatch SCR2496
M-audio Flying Cow.
Of these, I own the Behringer. It's very good value for money, uses AKM converters, 24bit 96Khz capable, selectable sample-rate and can do a few more tricks like stripping SCMS protection out and converting between optical or coax s/pdif and AES/EBU on XLR. However, it has no mic preamps being balanced line level over XLR connectors.