I2S cables should be kept as short as possible, .5M or shorter best. I2S was never designed to transmit via cable such that theoretical best position for clock is just prior to I2S path in dac. Ed Meitner of EMM Labs explains this issue for those interested in the whys and wherefores.
One should also purchase an I2S cable vs HDMI cable, you only need 8 pins wired to transmit data and clock. And if purchasing more expensive silver cable this can be money saving, silver values being what they are. I've also found quality of cable can make a meaningful difference as well. I've only had two thus far, but the Tubulus Ximius easily bettered a $70 copper cable I was using. Both Audiolund and Tubulus make more affordable cables than my Ximius, but may still be above one's present budget.
As for extra cost of ddc and cable vs using that money for a better dac, one can only speculate. I could make a strong case the ddc won't become obsolete as long as one continues using dacs with I2S inputs, it brings out full potential of these dacs whereas usb this not likely. My Musetec 006 has pretty much state of art usb board and I2S easily bests it.