Excavators: Improved is generally the way to go if you can get one. An adjusted technically is more eco than the TT version, but similar to overkill in the old hunting system, you generally lose what eco you gain on the last pull if it's only a few pec (i.e., would have been cheaper to use the TT exc. on the last pull). Speed is why you would upgrade from the TT exc. though since it takes many, many pulls to see any savings returns on buying adj. or imp. exc. The ROI on them is lower than UL amps I'm pretty sure. Going to modified or other high decay exc. can be a ped drainer though, so I generally say stick with TT -> Adj -> Imp and don't worry much beyond that. Unless you're doing PVP or very high mob density areas, additional speed doesn't matter much. Speed enhancers can be worth it for improving eco, especially on the Imp.
Amps: Anything around Terra 1 up to lvl 4 UL amps seem fairly priced from what I've seen in terms of being able to make an ok ROI. Below that, and you're better off just buying cheap L level 5 amps. Above that, and rarity somewhat maintains the old prices. I wouldn't say they're not worth at that point, but there are other routes that might work better for people depending on their mining style. If you don't want to futz with other stuff and keep things simple, there is a place for currently priced UL amps. I don't go that route though, but an UL level 5 or 7 could maybe temp me someday. The set of conditions where those prices really could work for someone are fairly narrow though.
Finder: Unless you still have a lot of skilling to do and can't use an L finder that hits the depth you want, UL finders + enhancers generally aren't worth it. That's as long as you can get the L finder under say 120% MU (ideally <115%). Finder decay does get mostly returned in claims, so all the matters is hitting the depth you want for as little MU as possible (enhancer or finder MU). For an F-106, L finders almost always win out, but an UL TM3 might be close.
Refiner: Only one mod exists to my knowledge, but if you can get a hold of an adjusted, it can be worth it. I'm not sure if any imp. versions exist yet. Imperium Resource Refiner B1 should be anyone's starting point for ~10-20 ped MU, T-105 after that, then higher end if they can get a hold of them.