An update since I just had this done this past weekend. I'm in the Seattle area, used a local smaller independent called Affordable Dry Ice Blasting that was indeed quite a bit more affordable (by about 50%) than some of the bigger outfits in the area. As detailed in another thread I have pulled my transmission for 5-speed swap and had decided to pull the engine to paint the engine bay, so it seemed like a perfect time to have all of this done. The underside of the car was completely caked in black grime that was very difficult for me to remove with degreaser and a brush. The result that the dry ice did is far superior to what I could have done, and for around $2k including underside, wheel wells and engine bay this isn't cheap but definitely one of those circumstances where the result for the amount of time it would have taken me definitely works out. It's a pretty impressive result and as mentioned above it's safe for work around hoses, undercoating, paint etc.
Here are some pre-photos
Here are some pre-photos
