You are correct, long analog lenses can perform really well on digital, I've seen that myself. Shorter analogs (say below 60mm or so) don't perform that well since the lens design then needs to take sensor glass into account due to the low angle of the incoming light, and there's also a higher overall need of correction which older designs don't have. I haven't done this particular comparison though I hope someone has, I'm also interested in the result.
I guess you mean the Rodenstock Digaron-S 180mm, there is a Schneider Digitar 180mm too which costs significantly less than the Rodenstock, actually about the same as the analog-based design Rodenstock Apo-Sironar digital 180mm. I'm not sure what the difference is between "Apo-Sironar" and "Apo-Sironar digital", the claimed image circle on the analog version is much larger though. Of all these the budget alternative if buying new would for me be the Schneider Digitar 180mm, digital and not more expensive than the analog counterparts. On the second hand market the analog lenses are generally cheaper though.